If the number of cases is small and can be completely enumerated, sure. But there are plenty of algorithms out there where the number of cases is effectively i

If the number of cases is small and can be completely enumerated, sure. But there are plenty of algorithms out there where the number of cases is effectively infinite -- the matrix inversion function someone else mentioned is a great example -- where it is prudent to routinely check results to make sure the algorithm is working. (Mind you, also having a unit test to sure it works on a small set of carefully chosen cases is a great idea.)