I've found where they live, how to switch them on or off…
https://apple.stackexchange.com/search?q=time+machine+local+
What I haven't found is – if in a Mac Pro with the Time Machine volume permanently mounted & scheduled for once an hour, is there really any point in these local files?
Do they prep for what's to be backed up at next run, or do they just scavenge changed data in case the scheduled task has to wait for any reason – drive not present, etc?
In short, do I need them in this situation or can I safely just switch it off & delete the existing files?
Best Answer
In short, no.
I've disabled local snapshots on both my work and home iMacs, both of which use a mirrored RAID for their TM drives. I also replace the Time Machine drive units from the RAID boxes every two years as they run 24/7. I use Guardian MAXimus from OWC*, which are easy to self service. Lastly, I make a quarterly DiskDupe backup of each machine to an inexpensive 246Gb external SSD (one each) and store them at the other location (work drive goes home & visa versa.)
For suspenders as well as a belt, I store my most crucial documents on iCloud, Dropbox AND Google drive.
This Apple Support document has interesting updates about how High Sierra uses local snapshots when compared to earlier versions of macOS.