I think some people go for reciprocation idea of saying out loud things like "I upvoted you!" to make the person feel obligated to give something in return, I feel like it's kinda shady to go around telling people you upvoted them for that reason, but I think it's ok for people to say things like 'great post', 'nice points you made', etc. so long as they actually mean it, when it becomes some habit to just go around giving super generic responses it's not cool anymore. I have seen on a few posts where it's a photo or some artwork and there will be a post saying something like "very informative post you have made, I follow you and upvote" and that's just goofy, a waste of time for the person uploading their artwork to see they have a response and find something like that.