My understanding is that tests are mostly supposed to increase iteration speed - the opposite of what most comments here suggest.
> The change curve says that as the project runs, it becomes exponentially more expensive to make changes.
> The fundamental assumption underlying XP is that it is possible to flatten the change curve enough to make evolutionary design work.
> At the core are the practices of Testing, and Continuous Integration. Without the safety provided by testing the rest of XP would be impossible. Continuous Integration is necessary to keep the team in sync, so that you can make a change and not be worried about integrating it with other people.
> Refactoring also has a big effect
- Martin Fowler