Sometimes a lot of replies can be somewhat overwhelming for new users (see this previous discussion). On the other hand, often times the immediate reply might not be the best one, or may happen to work for the OP’s use case but not actually be a good practice and cause problems down the road. Furthermore, while you may be satisfied with the initial reply, most threads end up getting hundreds and sometimes thousands of views, so they may be helpful for others.
Therefore, it seems useful to be able to mark a thread as “answered” for the benefit of members looking for answers, versus, those for unanswered questions to answer I’m not sure “mark as answered”—though that can also be done right now via a change to the title (“[Answered]”), or via an Answered tag (which we at least briefly experimented with, but it was hard to discover unless it reached a critical mass of usage).
However, I really don’t know about marking one particular answer as “the” answer, as the asker is typically going to be the one with some of the least expertise and experience to judge which answer is actually the best overall (if any clearly is, whereas there is often multiple covering different aspects of the problem); this is something SO has moved away from as well, as marked answers are no longer given priority in the listing after clear community feedback. I tend to find the number of “likes” an answer has to be better (though by no means perfectly) correlated with its approximate overall usefulness.
To note, if we did introduce such a feature, it would be a necessary pre-requesite to separate the “Python Help” and “General Discussion” categories, as also proposed in this related recent discussion, and earlier discussed at some length in a previous one that resulted in the current status quo, as those features would apply to “Python Help” but not “General Discussion”.