In the chess world, everyone—every strong player in the world, it seems—uses ChessBase, which is Windows only. They used to have a Mac version years ago but it apparently was the worst software ever. It's used for studying recent master games, storing and training on your opening theory, analysing with computer engines to find new moves in the opening, studying the games of your next opponent etc. Other programs have similar functions, but it's more powerful, and no grandmaster uses anything else. Also its data format is secret, so no other software can convert to or from ChessBase format. It was created by some Germans in collaboration with Kasparov in the 1980s, and was instrumental to his unprecedented level of opening theory knowledge.