I don't do anything too fancy. Just a vague "This is how they look like" headshot that's nice enough to post everywhere. I have a ton of notes and stuff that I keep to myself.
Personally, I don't think everyone needs to make one for their comic, but if you're just getting used to drawing the characters, it's a great help when starting out on a new comic. I think we all know that character designs can change in really subtle ways the more we work on our comics XD
But I will say, they make for some pretty great behind-the-scenes content.