> ??? emulators already had literally all of those things

Yes, but badly. Any old Amiga game that requires you to jump with Up or requires keyboard keys for certain functions becomes unplayable on a gamepad in an emulator. When you throw the game into the AI and do HLE emulation, you can fix it all with ease, and you can do it all context depend, e.g. a platformer level might need different controls from the spaceship levels or the menus. You can also add on-screen menus when needed.

For console games this isn't generally much of an issue, they controls map pretty well to modern gamepads and play well in emulation. But home computer games play absolutely terrible in emulation, as the controls are all over the place and each game has their own little quirks.