Not every Forum section has the 100 Character limit. As you may notice, some have 50, some have 10, and I believe the Life advice section requires a minimum of 200 Characters per post.
The awnser to your question is simple: To keep the Forums clean and tidy, avoid spam, and to have proper, worthy awnsers for each topic. A post should always be justified, nobody wants to see the same old and boring "No", "Yes" awnsers. It is often used to avoid advertisement on the forums, as links to websites, do not count as characters.
Concord's post in Why do we need to write 100 words in a post? was marked as the answer