What is the advantage of a custom rom?

[gpgorbosjr]

The question is in the post. I came across custom roms first when I was flashing the newest stock rom to my Mogul, and was going to try a custom one, but the Mogul's usb port stopped working right.

So, I was just wondering what the advantage is? Nobody ever explains it, and all I see in the logs are "removed X" "added X" which I could just do myself.