Mastering is absolutely necessity for cohesive audio across a variety of playback systems. Here is why:-
Audio Restoration: Converting vintage recordings as well as old and worn analogue masters to new digital remasters for preservation as well as to met the modern standards of mastering.
Stereo Enhancement: Stereo enhancement deals with the spatial balance (left to right) of the track. When done right, stereo enhancement widens the mix, making it sound bigger. It can also help tighten the centre image by focusing the low-end towards the centre.
EQ: Equalizing is a process that corrects any spectral imbalances and enhances elements that need to stand out. A well-balanced and proportional master means that no specific frequency range sticks out. A balanced piece of audio will sound good on any playback system.
Compression: Compression corrects and enhances the dynamic range of the audio mix and keeps louder signals in check while bringing up quieter parts. This process ensures uniformity in the audio mix. Compression helps glue together parts that might not be as cohesive as they could be.
Limiting: The last process in the mastering chain is usually a special type of compressor called a limiter. Limiters set appropriate overall loudness and creates a peak ceiling. Limiting makes the track competitively louder without allowing any clipping that can lead to distortion on smaller playback systems.
Bit Depth Reduction & Sample Rate Conversion: Sample rate conversion is dependent on the final output medium. For example, if the track is going to be released on CD, it will have to be converted to a sample rate of 44.1kHz with a depth 16 bits per sample and therefore, the audio file will have to be converted and dithered to get to the standard of that format.
Sequencing and Spacing: Sequencing and spacing is one of the final steps in mastering an album. On an album or EP, this process puts the audio in order. Spacing refers to how much silence (space gaps) is added between each track.
Saturation and harmonic distortion: The tendency of mastering engineers to use a lot of analogue gear results in added harmonic content to the mix which is often very pleasing to the ear.
Mastering is clearly a crucial aspect of the process of distributing music, mastering makes the track sound uniform, professional and balanced. Whatever the format the audio track is being released in, this process ensures that the sound is heard with highest fidelity and clarity while carrying over artistic intent.