One reason - to actually learn javascript.
It is a truly beautiful language that is often overlooked, because when people say "I know javascript", what they actually mean is "I know how to copy&paste $('#content').hide(200)
to make my app look cool".
This is a bit sad.
That being said (no pun intended), I use jquery all the time for mundane tasks like xhr, DOM manipulation and such. Unfortunately, there is absolutely no reason to reinvent the wheel (more than one time, anyway), especially taking into consideration the fact, that, at times, you have to invent a separate wheel for every major browser.