The forum is for discussions where you can reply and really get to the bottom of something if you have a problem or question.
The Answers section is just for relatively short answers that don't require further discussion. You can simply poll something and see what the responses are. It is also possible to see how good the answers are as others can vote them up or down.
When you ask a question you can only reply once to your own question but in a forum thread you can post as often as you like (until you get banned for spamming....).