If you observed something to work 15 times, how do you know that it'll work 16th time? You obtain a model of increasing precision with each test, that lets you predict what happens next, on a test you haven't performed yet. The same way, you can try to predict what happens on the first try, before any observations took place.

Another point is that testing can be a part of the final product: instead of building a working gizmo, you build a generic self-testing adaptive gizmo that finds the right parameters itself, and that is pre-designed to do that in the most optimal way.