Why even bother marking a post accepted on meta? It does not seem to contribute to anything, so why even bother?
For the same reason as you'd accept an answer on the main site. Basically, accept an answer if it solved your question.
If you post a support question, accept the answer that solves your problem by telling you how to do whatever it was you wanted to do.
If you post a feature-request or bug report, you might get an answer from a developer or moderator, either saying the feature/bug has been implemented/fixed or explaining why it won't be. In the absence of such a definitive answer, you can accept the answer you found most useful.
If you post a discussion question, then answers and the voting on them may be used in the future to determine community consensus and site policy, so you should accept the highest-voted answer, since that will be the one used to determine policy even if you don't accept it. See also Does a meta question have to be 'accepted' to be considered as a community consensus? Quoting from @Null's answer there:
Only the person who asked the question can accept the answer, so it would be impossible for the accepted answer to stand as the consensus unless it is also the most highly voted answer. It would effectively give the asker the power to stand as consensus. It would also imply that there is no consensus if the asker never accepts an answer.
Yes, it's often fairly useless to accept an answer on meta. But it serves the same purpose as on the main site: to mark a question as being 'solved' and indicate which answer was the most useful.
See also the main meta question When do you accept an answer on meta? Quoting from the accepted answer there:
You accept an answer when someone posts an answer that is acceptable in answering your question!
Seriously though, accepting isn't as expected here on [meta] and certainly less so for discussion posts. There's even an open feature request to not calculate the accept rate on MSO because it's so different. Typically I've accepted when either someone has posted an answer that answers my question or when a dev has explained why something is status-bydesign or is status-declined, etc. Otherwise, I don't accept.