In every professional recording studio you will find a compressor. Compressors have the versatility of being used during recording, mixing, and mastering. Lets start with mastering.
During mastering, a compressor will be used lightly to control the dynamics of the entire song as a whole. With this in mind, mastering engineers rarely apply no more than two to three decibels of gain reduction. When mastering a song, mastering engineers only have access to a group of instruments, (drums, vocals, keyboards, etc.) or, one stereo track with all elements of the song summed together.
Moreover, a mixing engineer has the ability to use a compressor in as many ways he or she can think of. While mixing, compressors can be used to control the dynamics of individual instruments, a group of instruments, over the entire song, and for effects all at the same time.
Furthermore, a recording engineer uses a compressor specifically to control recorded levels. Often vocalists can vary their amplitude while singing. A compressor allows the vocalists volume to remain consistent throughout the entire song or voiceover.