To create unity between the various tracks of a record and create a final product. When you’re mastering a full record, you can have individual tracks/songs which were recorded in a wide array of different environments and perhaps even with different producers. At the very least, every song will be mixed down to different levels. It’s the mastering engineer’s job to create a unified flow and relation between the tracks which make up a record to turn it into that final product.
To make everything on the record sound as good as possible. Tools like compression, limiters, and equalization can be applied to tracks to get the best possible sound even after the final mix has been handed over to the mastering engineer. Getting the best sound possible and overwhelming the client with quality is really the main objective.
To create the record itself and all that that entails. This means nitty gritty things like setting how tracks flow into one another, the space between tracks, setting the overall output (volume) level of the CD to be on par with other commercial releases, making sure the CD adheres to RedBook standards, and even adding information to a CD like ISRC codes.
In short, the mastering engineer has a lot of responsibilities and because they are the last ones to touch a record before it gets sent off to the CD replication plant, everything has to be just right; otherwise it’s a costly mistake.
To ensure every track is spectrally balanced, from sub-bass to treble, as well as creating a natural album-wide cohesion from track-to-track, so the album flows effortlessly.
To tie together the whole mix, bring out depth and detail, and occasionally rein-in unruly peaks.
To optimise track headroom, increasing the overall track level for a competitive loudness, limiting peaks without distortion or unduly affecting the track’s punch.