There's no real requirement for it, but the data can be read concurrently if the data to be accessed at a given point in time is on separate drives. Everyone seems to have theories on separating duties for their disks. I use one SSD for the OS, the apps go on another, and all my saved game data/general data goes on another. All I can tell you is the performance is great.
I have another machine dedicated to audio recording/editing/production by which Pro Tools recommends/requires a dedicated write drive for recording the raw data. You can actually notice the difference in record performance when using the separate record drive (less hicups in the recording).