Saturation can be a nice addition to the mix to make it sound a bit more full and warm in general. As explained in the Saturation section, saturation very subtly adds overtones to the signal, which can make it sound more full. It can also increase the perceived loudness of the signal, so this can also be useful to get some more loudness in your master. Like the other things in mastering, saturation needs to be added very subtly, to preserve the original nuances of the mix. I recommend to turn the dry/wet mix down a bit, so the saturated signal is blended with the original signal, to preserve the original frequencies a bit.