The answer to this question isn’t why -6db, but why leave headroom for mastering? Why would you want to leave gain available for mastering? Generally in mastering you are pretty much eqing and polishing your final mix with some compression and limiting/saturation and other fx and leaving headroom will allow the extra gain introduced into your chain from your chain some room. You need to understand why you master and why mastering adds decibels in general. The amount can be anything, some use -6, -12, -15 or whatever. These are just some numbers I have noticed some pros use. Any gain missing can be added back through a limiter at the end.