A protocol specification contains a large number of legal behaviours, and an implementation is correct only if it handles all of them. A hand-written testbench tends to exercise the ones the author thought of, which correlates strongly with the ones the design already handles.
Verification IP supplies constrained-random stimulus across the protocol's legal space, checkers that confirm compliance rather than merely absence of crashes, and coverage models that make closure measurable instead of asserted.
It integrates into a UVM environment as a standard component, which is how verification environments are built for anything of scale.