"One of the less appreciated benefits of test suites is how they speed up development. It seems counter-intuitive as you're writing twice the amount of code, but the experience of many programmers has been that test suites lead to better interfaces, ..."

Just to add one important benefit: quite often, when a programmer fix one bug, they introduce some other bugs that break existing functionalities. The test suites made it possible to quickly retest all existing functinalities, and quickly realize any of this type of bugs, thus lead to quick fix.