??
I only work on one song at a time. But when you do that your single drive is trying to
run OS
run program itself
stream multiple audio tracks from seq file (jumping around for the afore mentioned edits in that Temp Audio folder)
AND write the new incoming data to a new location on the drive.

That little arm on the drive may be jumping all over at times.
Reading is easier on the system (computers do it in almost 'real-time' all the time).
It's the writing that takes more time; especially if the drive is trying to keep up with reading multiple audio streams and then jumping to somewhere else to write the latest chnk of audio coming in).

You had the infamous MOTU setup. Think about what the soundcard did (play back 16 tracks to different ports and writing 8 more coming in to separeate tracks)
Now imagine a single drive having to keep up with that;

read 12 audio tracks, get buffer
jump over there and write that incoming data to different location
buffer empty, gotta go read some more
need to write my recording buffer NOW
jump back and grab some more audio

having another drive doing the writing is a load off a system at one of those 'pinch points'. The processor can keep up, but the bottleneck happens at the drive access.
This happens less as drives become faster (like RAM drives and SS drives), but it is still a likely spot for the whole thing to bottleneck for average user.
Having one drive stream and another write incoming data appears like a good thing to me for any user with the option available.

I do not work here, but the benefits are still awesome
Make your sound your own!