There are a lot of functional reason. I'll tell you mine...the 95% audio tracks need to be at the optimal sample rate. I know this issue gets religious, but that rate could be whatever--I will never do personal work at single rate again, but I get tracks from clients all the time at 44 or 48...sometimes 96...when I typically set and clock and optimize for 88.2. The workflow and results of VI work don't change no matter WHAT the rate is of the session. It's on a slave over THERE...connected analog to the recorder. I can optimize that "slave" to run the VI the way it plays/sounds best...and still not change anything about the main session rate. No changing the buffer to get less latency for a busy piano part...
Then there's the compatibility issue...I just had to work out some incompatibility issues with Ivory inside Mixbus. But, if I'm recording in Mixbus on the MacBook...and the "slave" is running Ivory standalone, they BOTH perform better, and there's no issue with anything. Logic's AudioUnit version have pretty globally been less stable than the mature VST versions for my instruments. Which isn't a dig on Logic or Mixbus...it's just pointing out the advantage in divorcing the instrument from the recorder. The LPX on my mac doesn't care if I run the VST version of Ivory on my PC...doesn't even KNOW...
Then there's the load time. When you open a Logic Project with all your RAM intensive VIs inside that project, it might take ten minutes to fully load and orchestral work...when you open project #2? 10minute minus some OS level caching speed. Where, with a slave, you take that 10 min once a day (or per boot I guess)...and you switch Logic projects in 10seconds instead of ten minutes. When you're working on say strings for 3 or 4 songs at once, like I tend to do--that adds up quickly.
When it became obvious that I was going to have to start using my old 64bit "slave" as an audio multitrack mixer, I went out and bought a Kronos...which really, served the same functions for the piano/keys as the "slave" had.
As to "how to"...I'll leave that for others more into the best current ways to implement that on a grand scale...it really depends on how/why you're doing it...and what two machines you're using, etc. Hope this helps.