Compression can make the difference between parts that fit together, and parts that fight each other. Putting a compressor — that’s the dynamics controller that squashes the difference between the loudest and softest parts of an audio signal — on a bus or auxillary track during mixdown is a vital mixing tool.
Very often, engineers will route similar tracks to the same bus so as to have a single fader to control them all. For example, a song may have twelve guitars, all routed to a single bus, so the engineer can then turn down all the guitars with a single fader, and not have to reduce the level of all twelve guitar tracks separately. Compression becomes an important part of that volume control. One guitar part might be an extremely quiet, delicate part, while another might be a very percussive rhythm guitar. Without compression, any volume adjustment could either lose the quiet part or make the rhythm track overwhelming. Judicious use of compression will bring up the level of quiet tracks and bring down the level of the louder tracks so that all the guitars sit in the same “place” in the mix.
Bus compressors are often used across the main stereo output bus of a mixer as well, to provide overall dynamics control for the total mix as well as to “glue” the mix together for a more cohesive sound.
Just remember to be careful: it’s easy to compress your bus so much that there is no volume difference at all between your quiet, delicate passages and your screaming leads. This not only sounds unnatural and fatiguing, but can thin out your parts and make all the instruments running through the bus sound dull and lifeless.