[foobarian]

Maybe it's slightly more convenient because the same value applies no matter how many cells are in the battery pack. So it helps distinguish the packs by electrical current output capacity which we can't get from a power value because power combines two different measures.

[notatoad]

mAh doesn't change as you add cells to the battery pack? i'm pretty sure that's not correct.

[samatman]

This depends on whether they're wired in series or in parallel.

Wired in series, mAh won't change, wired in parallel, it's additive.

[foobarian]

Most commonly RC battery packs are referred to as "1S, 2S, 3S," etc. indicating how many single cells there are in the pack wired in series. Those are also individually rechargeable via a low-current charging cable.

[Ancapistani]

Yep. I'm very familiar with these from quadcopters.

The charging cable is usually called a "balance plug", because it distributes power equally to all cells instead of shoving it all through all of them in series.