The reason is that if the code is the specification is not obvious when a certain behaviour is an intended feature or a bug. Specifications can have bugs of c

The reason is that if the code is the specification is not obvious when a certain behaviour is an intended feature or a bug. Specifications can have bugs of course, but when the spec and (potentially multiple) implementations agree gives more confidence on expected behaviour. A conformance suite can be an alternative of course, but it is not necessarily always possible to infer general behaviour from a test case.