Please Read - FORUM RULES!
If I compared a hair stylist to a mastering engineer, hair length would be headroom. If someone came in with 18" of hair, the skies the limit as to how she could style it. But, if they came in with only 1" of hair, her style options are very limited. In audio mastering, no headroom limits your options.
In the audio mastering process, a series of EQ boosts and cuts are performed. Most of the time you're going to need to boost something, even if it's only a little +2db boost at 100hz. Well, if the song is already at 0 volume level or higher, you might not be able to make a necessary boost without distorting.
Also, if a song has very low dynamic range (the meter barely moves) it's probably over-compressed. Which means it could lack punch, power, clarity, or could even limit EQ options. You want your song to have some dynamic life!
I want a song mix with some headroom and decent dynamic range. I want to EQ it as necessary, I want to compress it as necessary, I want to be able to set the overall volume as necessary, and I don't want to work with a distorted mix.