Testing is an absolute must for any non trivial (in size, or function) application that is to be actually used. You won't find **a single developer** who cares about his/her craft (as evidenced by their visiting of this site) that will respond and say that testing is not necessary.

In addition to what's already been posted, having a full suite of automated unit tests on any given application will make you more confident in future code changes. This higher confidence (as unit tests provide a BIG safety net) will result in faster code changes to existing applications (due to less backtracking/double checking)