ICANN participation

What am i paying the Registrar for and why can i not do it myself.

I have had a look around and found that domain registration charges change depending on the Registrar used. Why is this? are some doing more/less than others (what are they doing) are some just taking MORE profit for the same task?

personally i would be content with skipping the "Middle Man" in this and filling out required information myself, however it does not seem possible?

What requirements do ICANN make when registering a domain?