Why is mastering necessary?
In the right hands, mastering can transform a collection of good mixes into a great album.
Once a track is written, recorded, and mixed, the final step in the production chain is referred to as “Audio Mastering”
Not only can it seriously improve your recordings in a number of ways, audio mastering significantly improves the quality of the sound of your audio itself. The mastering engineer makes use of digital and analog effects to give your music more depth, atmosphere, balance and can even correct imperfections in the audio itself.
This is the most important aspect of audio mastering in terms of why you need to get someone with a good ear who knows what they are doing in a none biased way.
creatively, an artist can wait until mastering to edit songs, adjust fades, sequence the tracks and add the time between the songs. Generally speaking, the goal is to make it as easy as possible for the listener to get through the LP without needing to adjust playback, volume and EQ.