Compression at first too me a while to get your head around but the more I dove into it and the more I understood what the controls did/do and how it is shaped my music I understood how important compressors are. Originally Compressors/ Limiters was used to protect over modulation, this could occur when a signal that was being encoded onto the radio signal, A compressor automatically reduced the volume level when the signal went over a threshold, you can still hear this today on the radio when a presenter shouts or there is a loud noise, the compressor catches the volume over the threshold and reduces it regarding to what setting its has been set too, and then gradually or sharply comes back up to normal levels.
Below shows the main controls for Compression/ Limiters and a brief explanation on what they do.
CONTROL'S OF A COMPRESSOR/ LIMITER
Ultimately it's personal to how you use compressors, giving a overall reducing by 2-4 dB with a 2:1 ratio gives a gentle glue to a overall mix whilst a 15dB reduction and 10:1 ratio will give a slammed feel and bring out the tails in your mix, Its a personal thing.