When an emcee works in a studio they put a lot of effort into the recording of the music. After the project has been recorded they will always attend the mixing sessions. The mixing session is quite often a stressful piece of work because the band and the engineer must keep focus no matter how many times they must listen to the track. It goes without saying that if your paying by the hour small defects in the recording will go unnoticed, or just get ignored altogether.
Also some home recording studios may not have industry quality monitoring systems and less than ideal acoustics. This is becoming more common these days because of the large number of independent artists, and the subordinate lower budgets that come with them. This adds up to more small errors being missed such as digital clicks, bad audio edits, vocal pops, hissing, and other minor problems
The main role of the audio engineer is to listen for these small and annoying sounds and ensure the final project is free from these sonic mistakes. This gives the final track the professionalism and high quality of a project, allowing the audience to enjoy the track without distraction.
The mastering studio will have very high quality loudspeakers that have a full frequency response, or a neutral response not favoring any specific frequency range. These speakers should be set up in a very tightly controlled acoustic environment. In this room the engineer can hear deeply into the musical mix of the project. The engineer will be able to enhance the track and bring out the details. They create the punch and add a thicker more robust sound that will be able to be played to on other systems and headphones and still sound studio quality. Mastering really makes a noticeable difference in the overall sound of a project.
The experience of the mastering engineer means all of these little changes to the project will add up to a subjective improvement, making the song truly come to life. With the advent of the online mastering suite it is wise for recording artists looking for an audio engineer to know that there are a lot of websites that do not show their gear. I think this is a bit strange as an engineer is usually more than proud to show off their gear to impress a potential client. You should really consider asking why they do not show you their mastering setup.
Also check out their recent clients list so you know what you can expect. Who have they worked for? What are their credentials? How long has the person who is going to be mixing your project being working professionally? These are all reasonable questions to ask before spending any money with a mastering company that you find online.
It is highly recommended that you follow the mastering companies guidelines for uploading your project so the quality remains at an industry level. There are plenty of high quality mastering companies that can be found online, with a little bit of leg work you can find an engineer capable of making your project sound like it was produced by the majors.