Sorry to be late to the party, but this question got my mind working, so here is everything I could think up. Hopefully it's of some use.
Magic as a mundane force multiplier Suppose the warlocks are really good at some mundane skill. Giving them magic - especially if the magic didn't require years of study and practice since that would have been done by the witch - would make them that much better. Examples: Commandos who can infiltrate enemy lines, sabotage equipment, and carry out assassinations with strength magic or stealth magic / veterinarians who can speak to animals / day traders with minor precognition.
Team spells/rituals Many real world artisans work in teams, with one master craftsman and some assistants. Maybe magic is similar with some spells and rituals requiring multiple casters. The warlocks don't have to be masters, just proficient enough to do their part. Maybe everyone casting the spell has to have magic from the same source.
Protection against aging or disease Maybe men and/or women die young if they don't transfer magic. Worse yet, maybe they come down with a debilitating degenerative disease, like a magic Alzheimers or Parkinsons. Maybe without a transfer their mind eventually gets possessed by demons, spirts, alien intelligences, or magic itself.
Palace intrigue The royal family has two daughters, and of course the eldest one is going to get the throne. The younger daughter, or her allies, finds Joe Conman off the street and turns him into a warlock. Using his new magic to buff his appearance and charisma he becomes Courtesan McStudly. All he needs now are some official documents and an expense account and he's off to seduce the eldest. Options include making her elope with him, getting her pregnant, converting her to a heretical religion, luring her into a dark ally and stabbing her - whatever works in your world to disqualify her from the throne.
Covert operations At 10 am a warlock stealthily casts a spell causing a guild building to explode killing everyone inside. At 10:05 am, the witch who created the warlock reverses the transfer erasing all traces of her magic from him. The city investigators immediately try to track the magic to it's caster but they can't (the witch is in a different city, the warlock has no trace of magic on him), so now the only way to find the culprit is good old fashioned investigative work.
The transfer isn't unidirectional Maybe the warlock gets magic, but the witch gets access to something of his. His knowledge and skills? His senses? Maybe a witch can transfer fractional percentages of her mana into dozens of individuals. So the entire palace guard could be linked to a witch allowing her to see everything that is going on and help coordinate defenses.
Microtransfers Continuing the idea from above - giving some guy 0.1% of your power in exchange for access to his sight or hearing would be very useful. And who says the guy needed to volunteer. Get some enemy soldiers or dignitaries drunk, do the ritual, then sit back and take notes as they reveal the layout of the camp, their battle plans, their negotiation strategy, and everything else.
Puppets Maybe a suitably powerful witch can usurp the mind of the warlock and make him a puppet. Combine with forced transfer for tons of fun. Maybe trigger a self destruct spell and turn the unwitting warlock into a magic IED. Just one more way to get rid of that elder daughter in line for the throne.
Power transfer What if the witch creates a warlock and then has him gather additional mana and transfer it back to her via the connection.
Area of effect Create four warlocks and station them at the cardinal points of a city or castle. Then the witch can create a shield around the boarder of the square. Or maybe drop a plague inside the square. Or maybe the warlocks can function like pylons in Starcraft allowing other magical effects to happen within a certain radius of them.
The power of love Maybe a power transfer is like a wedding band and marks the couple with a visible brand (like a Bindi or something). This is necessary for the marriage to be officially recognized by the society. Combine with higher chance of magical children, or the ability to have children at all, as described by other posters.
Hive mind Not just a power transfer but the option to join consciousness as well. Maybe leads to better problem solving, faster learning and mastery, synchronized combat tactics, and the risk that the witch and warlock lose their individual identity.
Sacrificial spells Maybe magic is so powerful (or tainted by evil, or whatever) that many spells require the caster to sacrifice themselves or parts of themselves (why else would you have 2 kidneys?). Witches create warlocks to soak the damage. When they die, just make another. Combine with unwitting warlocks for a real dark scenario.
Releasing hidden power Some men harbor insane magical energy inside them, but it can't be used unless a witch transfers a certain amount of her power for a certain period of time. Maybe if Joe Accountant gets a 20% transfer for a year he gains the ability to Hulk out and smash his way through an enemy fortification.
Warlocks protect the witch (variant of some of the above) The universe itself doesn't like magic and actively seeks to punish witches for using magic. For some reason warlocks are immune to this and confer that immunity onto the witch.
Warlocks unlock the witch's full potential Although she gives gives some of her magic power to the warlock, the bond allows the witch access to some greater knowledge or power. Maybe witches can only cast Yin magic unless they have a warlock, then they get access to Yang type magic. Alternatively, only Warlocks can cast Yang magic, so if the witch's ritual needs Yang type magic to function she needs a warlock.
Magic affinities Variant on the above. Maybe a given magic user can only access a certain number of magic forms. Saraphina the witch is skilled in water magic and necromancy, and that's it. If she wants to use flame magic she needs to find a guy who has an affinity for fire. Not all guys do, so she has to search, bargain, bribe, seduce, and outcompete her rivals. Maybe some rarely occurring affinities lead to high status guys. If you're the only guy in the kingdom with access to earth magic, the ladies will literally be fighting over you. Combine with palace intrigue above.
Men without a connection become monsters (variant on some the above) After a certain amount of time, men not connected to a witch become malevolent and powerful monsters. Maybe they're scheming and cunning, or maybe they just rage out and start indiscriminately destroying things. Either way, they are a major threat. Now society has to decide whether to wait for them to transform and fight them, or encourage warlocks to be created (via bribery, threats, etc).
Magic as a drug Having magic flow through your body is addictive and messes with people the same as drugs (hallucinogens, stimulants, depressants, or effect varies with the person). Men pay women to transfer power to them for a period of time which gives them a magic high. Maybe women need to dump some magic energy onto men in order to reduce the psychological effects of having that much magic power within themselves. Lots of questions about how this society holds itself together. Maybe the drug effects only affect a subset of the population. Maybe warlock creation is mandated to occur when women are young and the drug effects haven't kicked in. Maybe there is some kind of twisted 2 child rule where each family has to have a boy and a girl. Or all births are twins (one male and one female).
I think I'm out of ideas for now. This was a very fun exercise.