Well, because it makes music sound better. Simple as that :)
Loads of people nowadays make music at their home, where there is no room treatment and so, the result may be very different from what they had in mind. The process of mastering treats your audio work with respect first and does all the changes required (no more, no less) to make it fit in the majority of audio reproduction systems (radios, car audio, home hi-fi systems, ipods, television, etc..).
You should consider seriously the Mastering process too, because your music is your child. You made it, spent loads of hours on it and you deserve a great result. Respect it the way we do and want the best out of it. We'll give it to you :)