Multiplying matrices and stuff is nice, but I don't think I really ever needed that. But simple combinatorics and other calculations that involve large numbers and lots of simple operations (like powers), but are perfectly calculatable without losing precision on a piece of paper — plenty of times. It's annoying that I can calculate on a piece of paper something my calculator cannot.
Other than that, maybe some operations over physical units/unit conversion and some embedded city/timezone DB, and I don't think I can ask more of a calculator.