people like to add their thoughts in the tags for multiple reasons I'd say
often times i for example don't usually add a comment in a text body but in the tags is because that way it stays as a nice tidy post on other people's dash. typically people don't want to reblog a post that has a random comment from someone on it, unless it's an active conversation or about sharing information. so if i just want to let the person who made the post as well as anyone who sees the post on my dash know, that i liked the art (for example) they will see it in the tags! and no one else has to bother going to a previous reblog or the poster to reblog the post without an added compliment

in short it's most commonly to not bother other users who want to reblog a post while also providing a comment to the people who might care (the poster and your followers)