It is very common for a client or rtist to spend a huge amount of time with their mix engineer to make sure that their musical project or album is sounding precisiely the may they want it to. It is generally in the artists best interest to make sure that his music sound the best it possibly can. But after hearing the same song over and over especially when its your own music, your ears can become used to what you're hearing. Problems within the mix will become "invisible" to you. This is where the mastering engineer can be of great help. The most benificial aspect of mastering, is the ability to get a fresh perspective on your mix. The mastering engineer will use advanced digital and analog tools to correct problems within your mix, while at the same time sonically ehancing it.
Online mastering has recently been gaining polularity, especially amogst independent artists. This is because online mastering studios have much lower overhead and can pas sthese saving on to the client/artist. Click here for online mastering rates.