So why doesn’t the mix engineer simply take on the task of mastering too? A mastered track should sound as good as possible on as many playback systems as possible, achieving a professional and consistent sound whether listened to on a car stereo, club soundsystem, cheap earphones, television, mobile phone, and so on. A producer or mix engineer has likely spent countless hours creatively blending multiple elements together to craft the final mix, and in doing so has ‘overlistened’ to the track in the same studio, which may not be the ideal listening environment. The mastering engineer is a final, experienced pair of ears that can objectively listen to the track, correct errors introduced by an imperfect mixing studio, and transparently sweeten a piece of music further.  
Fans and listeners are used to hearing modern music coated with a professionally-mastered ‘sheen’ - sparkling highs, deep bass, consistent frequency spread and dynamic balance. Today’s  
songs and albums must compete with other professional records. A skilled mastering engineer has the equipment, experience and ears to help a mix sit beside other commercial releases, adding that final 5-10% of polish.  
Collections of individual tracks - whether destined for an EP, album or compilation - can each sound tonally and dynamically separate from each other. The mastering engineer will ensure the final collection of songs all sit together as a single cohesive product.  
Having said all that, many of us do now choose to master our own tracks at home, so we want to help you to make the best job of it that you possibly can. Head on over to our Mastering week hubpage for tips, tutorials and advice from the pros.