FMS uses a disk cache, like filemaker client. Of course this cache should be the fastest possible.
Allowing us to, optionally, specify a preferred cache location, would allow us to use the best newest storage technology possible for it.
Especially as those new technologies are expensive, so have lower volume as the boot drive.
For instance optane drives.
Moreover, having the cache on a other device than the boot drive, in itself is better because, this extra device has not to compete with the os in I/o term. 100% of its i/o and bandwidth would be dedicated to FMS's cache