Gaming

They're a monopoly because they're the best in town and it isn't close. Steam has a(n easily circumventable and long, long-beaten) DRM available, but it's hardly the selling point of the platform nor a requirement. Steam provides automatic game updates, social and community help features, a built-in mod uploader/downloader, image hosting and a screenshot button, cloud saves, excellent network implementation (holy shit I do not miss the days of trying to get Gamespy and GFWL to let me connect to my friends), the Community Market and cross-game trading, built-in control remapping support, Twitch-style drops and giveaways from livestreams, recommendation features like Discovery Queue to find new and interesting titles, a community reviews and tags system that while imperfect is definitely the best I've seen thus far, an in-game overlay that has semi-recently gained some excellent features like game recording and the ability to pin a clock or notepad on top of the game, and one of the most recent updates reworked family sharing so that any five accounts can basically just merge their libraries and all play each other's games at the same time, an extremely convenient feature that far and away beats out consoles' digital sharing or traditional "pass the disc" borrowing.

Additionally, huge proponents of non-Windows gaming. Initially trying to do Mac ports, but moving onto perpetually pushing Linux as Apple continued to not care about gaming at all until like five minutes ago when they started paying Capcom and Ubisoft for ports.