Isn't it more predictable to know that you'll have at least (say) 500MB free, instead of gradually filling up your drive until one day you don't have any more space?
Edit: to be clear, it will actually remove backup data from the drive over time to maintain the amount of free space you set. It updates the central server to tell it which files you still have.
I have a 24TB disk array, configured as RAIDZ2; 12TB of usable space. When I run out of space, I add more disks. (Or, more usually, replace old ones.) I use Prometheus to keep track of its state, and have an email alert set up when disk space drops below a certain point.
The problem with a program that tries to maintain a certain amount of free space is that I won't know how much space there really is. I'd need to subtract the amount the archive uses, then.. add back in the amount I actually want it to use.. and it all ends up as a bit of a mess.
It'd be much easier for me if I could just set it to use 2TB, and be done with it. I could probably rely on it actually using the space.
Edit: to be clear, it will actually remove backup data from the drive over time to maintain the amount of free space you set. It updates the central server to tell it which files you still have.