Mausoleum of Cao Cao adds a concrete piece of history to Anyang, giving you something more tangible than dates in a guidebook. The value is in being on the spot: seeing how the building, ruins, district, or memorial sits in the modern city around it. It sits well outside the centre, so treat it as a deliberate side trip rather than something to squeeze between central sights.
It is worth reading a little background before you arrive, because many historic sites reveal themselves slowly. Give yourself time to walk around the edges, look for plaques or small exhibits, and connect it with nearby streets rather than treating it as a single photo stop.