I'm going to try to answer this from my beginner's background. I don't even know about the mastering process when I got started with Sonar - all I do was trying to record and arrange, not even mixing properly.
Then, I needed to do some mastering myself - after hearing complaints that my work was not as loud as the commercial one. So I jumped to Ozone / T-Racks for solution - I got two, to find out what are the similarities or differences.
Ozone offers many presets to choose from and I get to listen to difference results. I was also learning how to mix properly at this stage. As my mixing got better, I don't have to use Ozone much. I'm using T-Racks more, mainly for some limiting and brick walling process. All these will never be a true mastering work, just a more polished mix.
So, looking back, I can now simply use the Pro Channel modules and other effects at the group master to achieve some satisfactory results. However, Ozone / T-Racks play a big role for me to get comfortable to do 'mastering' task without them. ..But I'll still use them for ease use and setup.