Here are what I think are the main functions of sidekicks (and I was thinking about the Gilgamesh story too, but I think that was more a God/servant kinda relationship - like Moses and Aaron thing)
Moral support - some sidekicks, like Sam Gamgee, Ron Weasley, are there for moral support - a kinda surrogate brother/ advisor/ best friend rolled into one
Grunt work - some sidekicks, like Robin (Batman), Sam Gamgee do a lot of footwork, gathering information because they can blend in better than their counterpart (even in street clothes Bruce Wayne turns heads) - they do the servants work, cooking, cleaning up - think of the practical needs of the mission.
Brainwork - other sidekicks ARE the brains of the group - like Arthur from the Tick, Scully from X Files - they have to make sure the hero thinks things through at more than an emotional level
Comic relief - It might be too ignoble to have the hero fall on his ass and say the things that WE are thinking as readers, so we have people like Ron Weasley (in HP ) who do it for us. Comedy is vital, and even for the hero it provides an emotional support.
Can you think of other functions I may have left out?
Another observation I have while researching this paper is that people who can relate more to sidekicks (that is people who see themselves more as a Gabrielle than a Xena) absolutely detest the sidekick. Why do you think that is?