It can also alert you to ATC errors, which, although rare, can happen. For instance, on a recent IFR trip, I filed my flight plan based on the preferred routing I found online, but when I received my clearance it was slightly different. It added a couple of new victor airways, one of which was nowhere near my route of flight. When I queried clearance about the discrepancy, the controller agreed the routing wasn’t right, and so he gave me a new one — and guess what? It didn’t make sense either.