I find it useful to provide sources even to just make sure I don't misrepresent something myself. Many times when I felt like posting an outraged comment to disagree with someone, after looking for sources for my points, I just gave up because apparently, I was wrong.

Sometimes you just misremember things... sometimes you're just ignorant on a topic and what you've heard is all secondhand (and wrong). Providing sources is not something you do only for others, you should do for yourself.