The only experience I have with mastering is with Ozone so take this with a grain. When I have a mix at -6dB, I am able to 'paint more broadly' with the plugins within Ozone. I can go deeper tweaking the exciter, imager, EQ, Compression, etc. before putting a limiter/maximizer on the signal chain pushing the master to the proper dB. If my original mix is hitting zero, or close to hitting zero, I am unable to make any tweaks inside the Ozone plugin chain without the signal peaking. Does this make sorta sense? Again, this is just my super not professional way I use headroom for mastering.