Loudness
The loudness of a song is one of the main goals of mastering because the entire music industry is bent on competing in the form of, who's songs are the loudest. Well as you can hear in this case, all these companies are up for the challenge and no one is letting up.
In mastering we want to make shure the clients music is going be able to stand and compete commercially with the rest of the music out there. You don't want to have someone listening to the radio an they've just heard a couple of songs go by. All those songs they just heard had that loudness energy with drive and punch, and here your song comes on that has not been mastered with loudness an you sound weak by comparison.
Standing out like this with a song that is low or weak makes it sound like the quality in the recording is not there. That can have a bad effect on the listener that might turn them off to whether or not they continue to listen let alone buy your music. Loudness is not the end all be all of mastering however, but it is the by product of the whole mastering process.
During the mastering process some of the gain can be lost but at the same time it is replaced as the engineer moves through the process. In the final, the extra loudness is added because or if the head room is there to do so. Loudness is not simply turning up the gain, but giving the perception of a louder sound at lower set volumes.
Examples of songs that have that loudness added to them properly are when you listen to them even at a moderate level not cranked all the way to 10, you still have that loudness at a volume as low as 2.5.