It is always good to play your track in as many different environments and on as many different systems as possible. It's all good and well having your track sounding good on your system, but everybody else doesn't have your system or environment.

The Mastering person will be familiar with their own setup and its sound. Also, they would have a setup that is configured and an area that has been treated to be a critical listening environment. Additionally they will most likly have experience in getting a sound on their own system that will sound good on many other systems.