When looking at a topic, the wrench icon at the side lets you take actions on the entire post.
You can select one or more comments, including tools like “select all below” or “select + replies”
Generally, we want to keep threads on topic – it is easy to link and transclude other comments and posts, so we can keep threads focused and easy to find by starting new posts.
You can also easily edit “top” post categories, tags and their title by using the pencil icon next to the title, for quick fixes.
For instances, quite a lot of people post to the #how-to category not with an answer, but rather a question of “How To Do Something”. It’s quick and easy to change the category to Discussion, and maybe add a tag or two to make it easier to find.
There’s a discussion on guidelines for this.