Its also an excellent way to think about your app.

What are the core things your app should be doing from a data perspective? Expose those things in an API.

Making the API available over REST or as a Web service is extremely simple, and its amazing how useful it becomes over time in your own development.

It also makes you think twice about changing how these core methods change, which is often a good thing (especially when you are working in a small team and its tempting to make "little" changes to solve your current problem without thinking about its implications).

Cheers