03 Matching Sound... Even if all your tracks were recorded in the same studio, they may sound very different.
"If, for example, you have ten songs recorded at different times on different recording desks by different engineers, then the sound can vary from song to song."
Jason Goz
Mastering Engineer
Transition
Each track will have its own sound and each mix will have its own identity - some may sound bassy or may have more top end compared to others.
A mastering engineer will process the tracks to get a consistent sound.
The mastering engineer can tidy up things like fade-outs, trim down any overly long tracks and also put the right amount of space between your tracks.
Spacing is the sort of thing you don't notice when it's right but it can be really obvious when it's wrong.
The engineer can set the silences between songs to get the feel from one track to the next just right.