I can think of two reasons off-hand based on two reasons why one would have that in the first place. The issue is not coming up with a reason, but justifying it under the rules of the world.
If a person's capacity is to be relevant, then there has to be a reason that stems from either the progress of magitech, or from the laws of magic.
#1: Any One Battery can only hold one person's mana efficiently
While mana capacitors can be mass-produced and storage units are fairly common, people and businesses have run into the problem that after a person dumps mana into the device, another cannot efficiently add to it until the system has been emptied.
This makes some sense if you compare it to the physical world -- if you transplant an organ from one person to another, then there is the real likelihood of rejection. While putting a second person's magic doesn't cause a large-scale rejection, it does mean that an appreciable amount of power is spent making the new power compatible with the mana that is already there.
One could try to strip the person's life from the magic itself, but that leads to losses as well. It is a bit better, but not perfect.
However, if you have a person with an absurdly large natural capacity that refills fast ... then that person can fill a mana capacitor up fast with only one person's mana -- their own. There is no need to try to uses loopholes and workarounds in order to be able to get multiple people to fill up a reservoir.
No matter why this is, the point is that if one person can fill a tank of magical gas much faster than a team of two or three, then that person will be of value, if only as a better power outlet.
#2: Magitech still needs casters in some manner
This relies a bit less on raw inborn ability than it does for training.
While machines can laser-etch the appropriate circle on a sheet of purified copper, or precision burn the runes into an object, the root of the matter is that at some point a person has to actually cast the spells needed to activate the magitech. Once cast, a magical battery might be able to keep it going but the fact is that a battery for whatever reason cannot cause that initial ignition of a magical item.
For many mass-produced things like everglowing streetlamps, this might be done in a workshop/factory on an assembly line. Depending on the level of magic needed to actually initiate the item, having a large capacity might just mean that you can work for 8 or 9 hours safely as opposed to 4 or 5 for the average magitech technician
For this, by expending magic casting the initial spells, one is growing their capacity no matter how slowly it is happening. This would lead to some people having large capacities by the simple manner of working their magical abilities and getting stronger with them.