Why should you never have to explain why you're in agreement with something? What kind of culture do we have if you only have to explain why you disagree with something?
"Issue 1234: We need this feature because reasons A, B and C."
If you don't think that A, B and C justifies the change, send a downvote and/or explain why A, B and C aren't good enough reasons.
An upvote is sufficient to say that "Yes, I agree with A, B and C. Do it."
Of course, if you agree with the change but for reasons other than A, B or C, you can leave a comment to start a discussion. It has nothing to do with culture. It's just a way to minimize the redundant "Yes, I agree with what you just said".