Like a lot of you have said, my main reason for commenting is not knowing what to say. I get shy and worry that I'll say something weird, but I try to force myself to think of something to say if I genuinely like the comic, because I want the artist to know that and I'd want someone else to do the same for my comic. Even if a comic already has a ton of comments, it can't hurt to leave one more to let the creator know that another person enjoys their work!
As a creator though, I can say that comments generally aren't annoying, and you shouldn't worry about saying something someone else has already said. If two people say they like something about a page, that makes me twice as happy! Same if the same commenter says they like multiple pages. As a creator, it's so easy to forget that there are people reading your comic who don't leave comments, so even just a "nice!" or even an emoji can be a nice reminder that someone is enjoying your work.