>>>>Why would you need to leave headroom though, if the track isn't clipped what's stopping the mastering engineer from simply lowering the track to a desired level him/herself?<<<<
Because that is (part of) the mastering engineers job. The mastering engineer is supposed to apply a different set of ears than yours (a well-tuned, task-specific set of ears) to make sure that each track, and all tracks on an album, sound "finished and professional," and like they belong together. They are also supposed to make sure that your ultimate mix sounds great and translates to as many different formats and on as many different systems as possible (yes, you do this in mixing, but the mastering engineer has the final say, if you are working with a pro that you can trust). The nuts and bolts of executing this task is managing overall volume, track volume and fades (yes, fade-outs are supposed to be done by the ME), and overall EQ. Sometimes, a specific overall effect may be applied.
Volume management is a huge part of mastering. During the "Volume Wars" era (that we are just crawling out of at this time), it became de rigueur to have the absolute LOUDEST track possible (headroom and dynamic range be damned), and every mix engineer was pre-mastering their tracks with the files maxed-out to the hottest/loudest possible extent; each and every individual track, and the stereo buss, would be soaked in compression and limiting. Then, they'd send the track to an ME and say "Please make it louder." This drives most MEs out of their minds. Ask any _pro_ ME, and they will tell you the best thing you can do to prepare your mix to be mastered, is to make it _quieter_ to begin with, leave lots more headroom, and lay-off the 2-buss brick-wall limiting. This gives the ME something to actually work with, rather than arbitrarily limiting their ability by maxing-out the track before they even get to hear it.
If you want to save money and master it yourself, do that. But if you're hiring a pro, then let them master!
GJ