You can add and remove components dynamically. Think about an RPG where Pawns can have a lot of abilities. The Pawns and any other World Entities can gain new abilities and lose others, get temporary buffs debuffs and so on.
Each ability / buff may be complex enough to require several functions, a bunch of variables and callbacks.
You could cram all that functionality into a Pawn; doable but cumbersome, not modular and not scalable. There’s no reason for everybody and their mother to know about everything that could potentially happen.
With components, you get to choose which pawn has what components, and only when they are needed. Actor components also support inheritance.