Topic: punBB and/or Wikka

Ok...I keep seeing pages adding Wikkis.
Now, the whole point of my page is just the storing and exchange of random information - however, divided into topics.

Now, what are the ups and downs of a forum such as punBB and having a wiki.
I mean: If I have a forum, isn't it just a modified and user-based wiki?

So, why would I need one?
I am just really confused on this.