Delaying the write means you don't get an indication of the steps in the process; if someone is trying to debug a crash caused by corruption of the gc list (e.g. due to a bad extension module), deferring the writes means they won't see any output before the crash, where before, they'd see:

gc: collecting generation 0...
gc: objects in each generation:

and numbers for however many gc list traversals succeeded before the corrupt entry in the gc list was hit.