Doing your own hardware benchmarking serves two complementary purposes. Knowing how fast your current systems are relative to one another, and being able to evaluate a candidate new server via the same measurements, are extremely valuable for helping nail down where the bottlenecks in your hardware are located.
Second, the difference between reality and your hardware vendor's claims or reputation can be quite large. It is not safe to assume that your system is fast because you bought it from a reputable vendor. You should not assume a SAN is properly configured when delivered simply because it's a very expensive item and you were told it's already optimized for you. Systems are complicated, odd hardware interactions are inevitable, ...
