Why Mastering is Important?

Edit by Tenson: This was an advert for online mastering, however it is an interesting question. Why is mastering important? Is it only to help poor systems sound better by making the tracks less dynamic, or does it really help the majority of recordings?

I found when I recorded my own music that I tended to have different tonal balances across each recording simply because the songs were mixed on different days and when you compare them side by side you notice it more. A little mastering in the case of my recordings was to apply some simple tone controls (LF and HF shelf) to get a similar tone balance and adjust the track levels to match.

In many cases it seems to me that the best mastering is where the least was done, but maybe that is just the illusion of good mastering.