https://hyperkin.com/Retron5/ (it was pointed out to me they violate open source licenses, so forget this one.)
every game system's "virtual store"
wrapping old games around an emulator to make them (seamlessly) compatible with current computers. Allows the publisher to continue to sell decades old games with vert minimal effort.
I don't know how much rereleases of games use emulation vs rewrites, but I assume at least some use emulation.
I believe I heard that the xbox360 contains an Xbox emulator for backwards compatibility. If it wasn't the Xbox360 it was some other console.
Then there's the failed bleem! commercial emulator.