There have been many film critics in the world, but no one seems as influential as Roger Ebert. If you check the wikipedia page of any movie, it's very likely that, in the reception section, they would have mentioned "what was Roger Ebert's reception to this movie". If you just google any movie, on the right hand side of the page they show a brief snapshot, where again it is highly likely that you will see Roger Ebert's score for the movie alongside Rotten Tomatoes, IMDb, Metracritic etc.
So Rotten Tomatoes, IMDb &and Metracritic show aggregated results, but Roger Ebert is just one man. What makes him so special ?