Why get your tracks mastered?
The job of a mastering engineer is to try connect the listener to the artist.
Mastering is the last step in the production line before the music is released into the world and it primarily involves preparing and optimising mixes for release onto the many different platforms/formats in todays music consumption world, so that it sparkles in the best possible way for the listener.
Mastering ensures the correct balance is achieved in terms of tone, dynamics, stereo imaging and loudness . In terms of an album or collection of works, it ensures consistency in these elements from one piece to the next so that it flows seamlessly for the listener.
It also involves quality control, ie fixing any existing problems with the audio and also anticipation and prevention of any potential problems that might occur with the various streaming platforms and broadcast etc, which is essential these days as there are many different specifications for different platforms.
Automated Online Mastering Services Vs Human Mastering in Studio
In an automated online mastering system, in general a track is uploaded to a website and comes back altered or 'mastered' matching the specifications determined by the designer of the system. Generally these systems make level and tonal adjustments to the track. What comes back may be of value to you but also the machine may not have the best possible answer.
The process is certainly not a bespoke version of mastering. The automated system won't make any creative decisions about dynamics, tonal balance or the feeling of a track, unlike an engineer in a studio.
Also because of the 'one size fits all' approach of automated systems, the dynamic impact of a song might be removed as it seeks to level a track in its entirety and not 'think' about the best approach for maximising dynamic impact from quiet parts to loud parts of a track, or indeed maintaining a dynamic consistency between a sequence of tracks on an album. So the dynamic contrast that was part of the artistic vision for the track might be somewhat lost.
Music is creative and there is still definite demarkation between between automated systems and creative decisions and the context they are made in.