... is the most upvoted stackoverflow answer.

Depending on the carrot, the skin can be significantly more bitter. And sometimes peeling can be quicker than trying to scrub dirt out of particular lumpy carrots.

YMMV

I tend not to when cooking for myself )unless it's been in the fridge for a while and the skin is a bit unappealing, no pun intended), but some people prefer carrots peeled for aesthetic reasons.