Motivated self interest.
In my case, sole developer translates to small business owner. I've written a reasonable amount of library code to (ostensibly) make my life easier. A lot of these routines and classes aren't rocket science, so I can be pretty sure they work properly (at least in most cases) by reviewing the code, some some spot testing and debugging into the methods to make sure they behave the way I think they do. Brute force, if you will. Life is good.
Over time, this library grows and gets used in more projects for different customers. Testing gets more time consuming. Especially cases where I'm (hopefully) fixing bugs and (even more hopefully) not breaking something else. And this isn't just for bugs in my code. I have to be careful adding functionality (customers keep asking for more "stuff") or making sure code still works when moved to a new version of my compiler (Delphi!), third party code, runtime environment or operating system.
Taken to the extreme, I could spend more time reviewing old code than working on new (read: billable) projects. Think of it as the angle of repose of software (how high can you stack untested software before it falls over :).
Techniques like TDD gives me methods and classes that are more thoughtfully designed, more thoroughly tested (before the customer gets them) and need less maintenance going forward.
Ultimately, it translates to less time doing maintenance and more time to spend doing things that are more profitable, more interesting (almost anything) and more important (like family).
