Well: in many cases, compactions can make more files eligible to be garbage collected. So if you want to accelerate the freeing up of space, running a compaction may serve that goal. It will temporarily increase usage, but once the garbage collector catches up the net effect is to reduce disk usage. Unless I'm mis-remembering how things work. (Caveat: I left Google almost 9 years ago.) And so "run a compaction to free up space" is a reasonable shorthand most of the time... unless you're so close to the edge that you don't have room for the temporary increase.