Here, we are talking about the processing done on your finished stereo mixes to prepare your album for commercial release. We discuss techniques, equipment, philosophy, and costs. If they are intended for sale to more than a local market, its a good idea. Consumers in general are used to high quality product from major labels. Without exception, this material has been mastered, to help produce the required sound.
People buy music that sounds the best.
Mastering is a different listening process, which takes time to learn. You tune in to different things than you do when mixing. The second advantage is equipment. Sound Forge, ProTools, Sonic Solutions etc. provide the basics for mastering, and the cost is reasonable. While this is attractive for a studio that wants to generate extra revenue, as you move up the scale toward major-label releases, you find most of the work happening at facilities with more specialized tools.