If you approve someone, you don’t need to explain yourself, you would just say “I agree with this guy”. There’s no substance to it.

However, if you downvote, you are saying “this is wrong”. Which is much different. When you accuse someone of being wrong, you should explain yourself, otherwise you’re being a dick.

It’s fine if someone already answered with what you were going to answer. You can just upvote that guy and move on.