A Pawn is useful in ANY game; I think the documentation is just saying that possession (the process of assigning a controller to a pawn) doesn’t work on client PCs within a multiplayer game (i.e. it’s up to the server PC to assign the controllers to the pawns). In a single-player game, the PC you’re running the game on IS the server, so it absolutely applies!
Like everyone has said on here, a Pawn is simply an Actor that has functionality built in out-of-the-box, saving you from having to implement it yourself. If you want to make a character in your game, you create a Blueprint class using the Pawn class as a base, and then extend it with your own functionality to make it behave exactly as you want. You could even look at the Character class, which is a special type of Pawn with functionality built in for walking, running, jumping and swimming!
So, in short, a Character is a type of Pawn, which is in turn a type of Actor.