If flying doesn't have benefits of speed or efficiency, and doesn't allow you to transport more than a single person can carry, that leaves one main advantage that I can see: the ability to avoid obstacles on the ground.
At it's simplest, this type of flight could allow you to move more quickly over especially rough terrain, like mountains or thick forests. However, that's probably not enough to justify the difficulty of learning the magic. If you increase the danger and/or importance of the mission, though, it would be important.
For example, you could use a few skilled fliers to transport food and other supplies to a beseiged castle or city, allowing them to survive and withstand the assault. By the same token, they could be effective at damaging enemy supply lines. Fliers could also be used as messengers in wartime, traveling on foot and using their flight to evade capture if necessary. Depending on how high they can fly, they may not need to even worry about arrows at all, or they could protect themselves with a lightweight shield, like one made of rawhide or wicker.
Of course, flying will probably make people more visible, so messengers would need to travel a path that is hard to follow on foot when escaping, crossing rivers and ravines or going up or down cliffs to put distance between them and their pursuers. There are situations where the visibility is an advantage, though.
When in battle, your officers could use flying men as signallers. Being airborne, they would be even more visible than they would be on a hill or ridge. Allowing your generals to communicate orders more effectively is a boring but very significant military advantage. From an altitude, the fliers would also have a larger field of vision, so they could be used as scouts. Having a better view of the battlefield allows the commanding officers to make more informed and better tactical decisions, another boring but substantial advantage. Of course, depending on what other magical capabilities there are, they could just drop fireballs on the enemy formations.
People with the ability to fly could also put their skills to use in construction. Cranes existed in medieval times, so they would only be of marginal use in building tall structures, but they could work on them more safely, or do minor repairs without needing to build scaffolding first. They would be more useful in bridge-building. It's a lot easier to transport materials for a bridge across a river or ravine when you don't need the bridge to do it. A single flier could probably build a simple rope-bridge in less than a day.
Overall, the limited type of flight that you describe has limited, apparently mundane, use. However, in a medieval society, the advantages that it provides would still be very useful.