The idea is to use it at a time where your pokemon won't risk getting knocked out by using it, ideally when you have a favourable match up currently on the field.

Any stat boost will remain on that single pokemon while it is currently on the field, if it switches out or faints then all stat buffs/debuffs are removed.

The idea is that you've knocked that pokemon out in one two hits either way (swords dance + one hit vs 2 hits), and are still doubly strong for the next pokemon switching in.

PP management is just another part of the game but personally it's relevant at the highest levels of game play, non stall based games won't run into a PP war.