FWIW, using a ‘Unit’ (as it’s referred to above) for a component’s power pins, is not a uniquely KiCad construct.
It is not rare to find such things in Altium libraries as well. There are good reasons for it, and good reasons against it. The person responsible for a project’s library management should decide, and stick to one strategy throughout.
My personal strategy for identical-unit-multi-part-components, which likely works best only for me, is to put the power pins on Unit A. Then, when placing Units B, C, D, …, be sure to keep space around all of the sub-parts in SCH such that B could be potentially swapped with A (or any combination of A, B ,C, D, …) in order to make routing in the PCB more efficient - once we get to that step.
This strategy also forces me to manage bypass caps appropriately.
There are downsides to the above method as well, so YMMV. Choose what works best for you, and do what’s necessary to provided clear and concise schematics.