All of my systems are Linux, launching Windows games on Linux is not trivial, Steam takes away almost all of that complication. It also provides an excellent ten foot interface for me to use on my TV and buy/install/launch games from my couch without any hassle. Speaking of controller usage, Steam provides excellent support to remap controllers even if a game doesn't support it, and provide amazing features at that (for example multiple layers, gyroscopic mouse)
Games getting updated automatically is a great feature, I still remember having to download patches and applying them one by one after a fresh install. Similarly Steam also provides a workshop that allows you to install mods and keep them synced across different systems automatically.
Finally, the convenience of cloud saves for someone with multiple systems or who uninstalls a game and reinstalls it later is not easy to achieve without a launcher (I still have a saves folder backed up somewhere from before).
Besides all of that Achievement and other social features are important for some people. And for some games being able to easily play online with friends is amazing (if you're not old enough to know what GameSpy is you don't know what it was back then), although I don't play too many online games so this one is not that important for me, but when I need that feature it is very handy.
In short there are many reasons, but if you're playing old single-player games with mouse+keyboard on only one windows PC, then none of my reasons apply to you. Still I would argue that buying games on steam is easier than pirating them, so there's the convenience factor still (e.g. at a friend's house and they mention a game, open my phone, and in 5 min with a very intuitive flow I have the game downloading on my home PC so when I come back it's ready to play).