These can be really really useful to help other people find solutions to frequently (or infrequently asked questions). Certain subforums allow you to specify a thread type as Question. Just click on the Question tab when creating your thread.
If/when somebody comes up with the solution, you just click on Mark as solution under their post which then becomes highlighted and shows immediately under the original question.
Note that that this is only for questions that may have a specific answer. If you want peoples' opinions (what's the best saxophone? What's your favourite song?) or recommendations choose Discussion or Poll
Anyone can also vote up any answers they think are good, even if there isn't (yet) a solution. Only the OP (or admins) can mark a post as solution