Well, a vocal coach can help you to get the most out of your voice. Teaching you different techniques, and how to access the three different voices (chest, throat (?) and head/whistle).

They can show you how to do a proper vibrato (which is actually one of my pet peeves in a singer-- when they do a throat vibrato instead of a stomach vibrato, and they feel the need to quaver every single stinking note).

Just like with a guitar teacher-- a vocal coach can help you do things you never thought you were capable of doing. Also, you can ask to learn specific songs and they can walk you through them.

It probably sounds like I've taken vocal lessons, but I haven't-- yet. I will, one of these days, when I get my act together. All the above advice is based on the internet course I bought, as well as advice from other people I know, coupled with a healthy dose of common sense. Oh yeah, and I guessed a lot of it, too.