I don't even know much about it and I'm a noob too but from being on this site for a few days(nonstop), I know that having a cooked rom is waay better than an OEM rom. Im gonna guess on this one(and guys correct me if im wrong) but I believe the general idea is too apply applications or have "custom" apps installed, unlocking things such as GPS capabilities, making more memory, faster speeds, cutting out all the crap that your carrier has implemeted in them. Plus it'll give you a new updated look and the sweet thing is you can change(flash) as much as you'd like.
(im pretty sure more responses will clear this up for ya.) 