A beaker full of death wrote:
what really is the purpose of the sidekick?
That's a great question, and probably deserves a thread of its own, but I think most sidekicks can generally be divided into two main groups: the greek chorus or the comic relief, which roughly correspond to the similar division you identified in your post.
These roles don't have to be filled by a specific sidekick; any character can temporarily fill that role depending on the scene in question. So I don't think a hero necessarily needs
But it does add some narrative ease/efficiency to have a close, permanent companion as that permits a greater exploration of the psychological and emotional state of the story using two characters whose mental states are already well-known to the audience (hero & sidekick). I suppose this is especially so in media where many characters are pretty thinly sketched anyway, so using pre-existing well-known ones enables some degree of depth when there just isn't time/scope to develop it otherwise. One might also argue that this is part of the reason for a tendency to mine a pre-existing & well-developed Rogues Gallery of villains.