It doesn't seem to be in the News Guidelines any longer, but the prevailing opinion on HN is that comments that simply say "I agree" or the like:

1) Add nothing to the conversation. Just upvote if you agree; also commenting to that effect adds nothing.

2) They clutter up the comments, so they are actually worse than not commenting at all.

I seem to recall a previous version of the style guidelines actively encouraging us to downvote such posts in order to train newer accounts to stop doing it. But I might not be remembering correctly.

In any case, I can pretty much guarantee that's why you were downvoted (and I didn't downvote you, personally).

Which feels a bit dated now. Expressing that you agree isn't only valuable to the writer of the comment, and since the number of upvotes-downvotes isn't shown anymore the only way to express that you agree is to write an "I agree"-comment.

Yes there are advantages and drawbacks of showing the upvotes-downvotes but one drawback is that you create a need for "I agree"-comments.

you have to be careful with downvoting of new accounts, if they make a useless first comment and get heavily downvoted, the account will end up hellbanned, which is extremely frustrating for a new user.

Creating an account to post a useless comment does not bode well for the quality of that account's future contributions.