From what I know, I can legally protect my software source code from being used elsewhere (even in a modified form), with an appropriate software license. This requires no long legal process; I just have to include the license in a few places.
At first glance this process seems sound to me; I don't see a need for a patent because it seems to accomplish the same thing via a longer process. On thinking a bit more, it is evident that licensing and patents are different1, but I'm unclear on how. Could someone explain this?
1. This may be a bit broad. In such a case it is OK if the scope is limited to software only.