When mixing, it is important to start with clean audio tracks.
Every time you add an effect, the noise floor is being processed too. In the case of reverb and delay you can end up magnifying and sustaining noise. In the case of EQ, compression, and saturation effects, excess noise will contribute to unwanted audio artifacts. During the mastering stage all sins will be revealed as compression levels are cranked up to compete with the 'loudness' levels common in modern music.
A common place to start is carving out silences (space between captured audio) manually or with the use of a gate. An expander can be a great tool in some situations as you can lower the noise floor during the actual signal. This is also true with noise reduction software/plugins, but I personally am reluctant to use that kind of processing so early in the mix. When it is necessary I use it very delicately.
In short, a little clean up on the front end can pay huge dividends on the back end.