Generally, NPCs are used to help generate plot and as adversaries. Frequently games will have “set scenes” or “set encounters” which the GM has designed in advance to serve up a situation to the players.
But some games try to steer away from NPCs and just have players interact with each other. This is great for the impromptu aspect of the game, but it often means that the special effects that can be applied to the situation are often not as good as a set scene planned for weeks in advance.
Should players plan “set scenes”? This often means they’ll be planning a course of action before the game starts and it isn’t strictly impromptu. Does this matter?