One of the most discussed topics in the music production is the purpose and necessity of using saturation through the whole process. Saturation generally is a gentle type of distortion that adds new harmonics in the frequency spectrum while may apply some soft compression to the signal. More generally we could say that saturation adds an analog feeling.
Using saturation while mixing or mastering in the box today is a kind of art. Technology has gone so far and the saturation tools we have in our hands are great. Almost any analog modeled plugin adds a touch of saturation. We could hear that in channel strips plugins, tape simulators or analog modeled equalizers etc. There are some plugins that have been created for this purpose like Hornet Plugins Graffio which is a multiband distortion with saturator, exciter and bit reducer and Decapitator from Soundtoys among other.
Using saturation is like adding the right amount of salt and pepper, there is no such thing. It is all about personal taste & favor.