Because this stuff dries FAST and HARD! It is perfect for that first coat of filling in old hardware holes. I always to do a good first coat of this, getting it deep into the holes, not worring about it looking good yet, and let it dry, which does not take long.
I have learned that there is no sence in trying to get the holes filled in perfectly with one coat of any type of filler. It usually sinks in a bit or has air holes. Best to accept the fact that it is a two step process and do it right.
This nice clean edge is easier and faster to sand out evenly.