While a test suite gives you a collection of examples of use, it doesn't tell you the intended purpose and motivation for the software, why it is designed the way it is, provide any argument for correctness or best-, typical- and worst-case performance, or discuss what trade-offs were made, and why. It is often hard to divine the underlying rules from a set of examples, and they don't go out of their way to point out which are the corner cases. There is a great deal of information in the design of any non-trivial piece of software that is not efficiently expounded by tests.