As another example, it's how QuakeWorld (and some other modern improvements on Quake) work. You're supposed to have a licensed copy of the game for the non-red

As another example, it's how QuakeWorld (and some other modern improvements on Quake) work. You're supposed to have a licensed copy of the game for the non-redistributable content, but the code of QuakeWorld is modern, runs better, etc. as opposed to trying to install Quake 1 itself from Steam and hoping that it runs.