If your DNS is behind a firewall, it can't reach the Internet. So it can't fulfill one of its main tasks, which is to provide name resolution. By pointing to a forwarder (often a "caching only" DNS running on the firewall itself), the problem is solved.
Another situation for using a forwarder is when a DNS is located at the end of a slow link. Instead of doing name resolution itself, all the queries will be sent to a forwarder at the other end of the link. Often the second DNS has a higher usage than the first, and, thus, already has a greater knowledge. It is in a position to answer the query immediately using its large cache.