It lets you directly jump to a specific character without bothering with movement keys. Basically what happens is, suppose you have 5 x's on your screen. You'd bring up ace jump (or its replacement, avy), and then hit x. It'd convert the first x to a, second to b, ... fifth to e. Suppose you wanted to go to the fifth one, you'd then hit e, and your point would move there. It's kind of cool. I have it installed, but truthfully don't use it very often.

Edit: Oh you meant "why would you use evil mode". Disregard.