There are 3 main pillars of contemporary stereo buss processing.
1. Reducing the dynamic range of a mix.
3. Colouration or saturation (people may use the term ‘glue’)
These usually happen in varying orders, there doesn’t seem to be a consensus of a hard and
fast rule in that regard. There are also crossovers within these separate elements, for
example some compressors that are less transparent will add colouration, or perhaps
change the mix tonally as perceivable EQ adjustment.
In short, compression and limiting is used on a mix buss to control the dynamic range. It is
to make the perceived volume of a song louder and also to provide a cohesion that marries
everything together. Equalization is used either for surgical removal of certain frequencies
and also shelving. The latter being primarily performed in the analogue domain, the former
occurring digitally sometimes even via a dynamic EQ plugin. Colouration is usually open to
interpretation, all the way from tape simulator plugins to outboard standalone transformers.
There is an interesting case of a Brisbane death metal band that once used
two separate JCM800s into PA speakers, with mics in front as the stereo buss! Recognised
as one of the most unique and remarkable heavy records of late, unbeknownst to those
working in a technical field that the process of mixdown was completely absurd.