Mastering has a lot to do with making all the songs on an album sound similar.
Often a record is recorded over a period of a few days or weeks.
The finished product the Producer hands over to the mastering engineering may have problems such as:
- different songs mixed slightly differently, making the album sound a bit incoherent, more reverb, les reverb, more bass, less bass from song to song.
- different songs mixed a different levels, so the mastering engineer will adjust levels from song to song so they all have the same volume.
- different songs compressed more or less than other songs
the record company (or producer or artist) may direct the mastering engineer to compress the whole album to make it as loud as possible.
The mastering enginer usually has the advantage of always mastering an album in the same room all the time, i.e same acoustics, same speakers.
The producer may work in different studios for different artists (or even different songs), so the producer may not really know the acoustics of the studio he has mixed in the way the mastering engineer understands the sound of the room the mastering engineer always masters in.
Really the mastering engineer is a final set of ears listening to the album before it is transferred to CD (or other format).