How is this better than sounding alarms when free disk space drops below 8GB? If you’re going to ignore the alarms, then you’re going to have the same problem after you remove your spacer file and the disk fills up again!
On the other hand, if your monitoring was set up well, you got a notification and had time to react to it before it was at 100%.
Granted, if you have a process that just wrote a file at maximum speed, that time window is tiny, but that's not usually what happens in my experience. What happens is that something starts logging more and it slowly builds up while you're happy that your server is running so well that you don't need to pay attention. And then the alert comes and tells you that there's less than 10% space available, and you have plenty of time to investigate and avert the crisis.
You solve the issue right then and there. Step 1. realize there is a space issue and get to terminal Step 2. free space so any solution has memory Step 3. Solve by doing <??? specifics ???>
Another way to think of this is that you have the 8 GB spacer file, but when the disk fills up the spacer file is automatically deleted and your alarm goes off. Which is literally the same as having your alarm go off when free disk space reaches 8 GB.
Also, forcing manual intervention has a psychological effect. An alarm that goes off at 8 GB remaining? Eh, I'll get to it at some point. A "disk is full, error, error, everything is broken"? I will deal with it -right now-, especially since I know a fix. Do that, with an alarm at 16 GB (so I still get the early alert in the event I'm that good a citizen and actually prioritize getting to the bottom of it even though it's caused no issue yet), and I'm in a better position still.
If you have an 8gb spacer file, at some point the disk will get full and cause some errors, so you will have to log in, remove the spacer file and then deal with the problem.
If you have an alarm for 8gb remaining, you will receive the alarm before any application ever notices the disk is full. You will have basically the same amount of time to solve the problem for good, but if you're able to solve it before these 8gb also ends, you won't have to deal with any "app crashed/misbehaved because it reached a point where no space was available" issues.