Mmm... Take my opinion with a grain of salt, since I frequently add (often translated) commentary into my uploads because most of the time it's pretty quick and easy for me to do. I am a little guilty of adding "useless" commentary sometimes though, but it's here and there... I don't consider it that harmful.
Going to add to what Sacriven and Mushroom said, although it isn't much. I add it because in addition to already said reasons, I think it adds a sort of 'life' to an artist's artwork -- like while they draw they have passing thoughts and often include those thoughts into their work. Mundane things about their day or occurrences or just being thankful to their followers/supporters, but things that help artists, curators, and users all connect to each other and start a small conversation, or at the very least spark something in the mind. Here's just some of a number of my own uploads:
post #2570151
post #2567468
post #2566743
The times where I don't add commentary are when I don't think it provides that much, or isn't really of much relevance (as they've echoed). I actually removed the commentary/commentary request from post #2553349 because that mostly consisted of ingame world information that most people who like the image won't be that interested in playing (and if they are, they'll probably find it on some wiki or something).
If I think of more reasons why I'll probably update this post, but this is what I have for now.