Mastering audio for a physical release is much more involved than simply having your audio mastered on a track by track basis for digital release, for example. The primary difference is the extra steps which go into creating an actual album out of your music versus simply enhancing the audio by itself.
When mastering audio for both a physical as well as a digital release, you’re going to add plugins such as EQ to achieve the most from the audio itself. In the hands of a skilled mastering engineer, these simple audio editing tools can give your music a wholly different and more professional sound even if you didn’t record in a professional setting.
When mastering audio for a digital release that’s where the process ends. The purpose is to get the best possible sound out of your audio. When mastering audio for a physical release such as a CD or a vinyl release there is much more involved. One example is that the mastering engineer creates the sequence of the record to ensure that the table of contents is correct and setting the spacing between the songs so that it’s to the request/specs of the artist.
Perhaps most importantly in this stage of mastering is ensuring that every song is at a similar volume level with every other song on the record. This is important because it’s likely that you didn’t record every song with the exact same settings or even in the same environment which can cause certain songs to be naturally louder when rendered into a final mix. By placing every song of the record side by side, the engineer can determine if particular tracks need to be boosted. In the same vein, the engineer works to make the entire record on par in terms of volume with other current records.
Ultimately, mastering audio is far more involved than simply improving the audio and really the engineer works to compile the image of that record and ensure that everything is perfect before it is sent off for replication.
If an easily overlooked error occurs such as one of the tracks not starting when it should on the compact disc version of the record, that’s obviously a costly mistake if left unnoticed as odds are the CD replication plant isn’t going to notice that problem and will press your records as is, rendering the lot worthless after you have paid.
This is just one example to serve to show that the engineer needs to be meticulous in every aspect of audio production.