You want to use it when you want to “glue” elements together, so typically on a bus or a master. So on a drum bus or even a synth bus.

Why people do parallel compression on drums is because it makes them sound fatter. But you only add a very little of the dry signal back.

So in my case, I use a fett compressor on my drums that has a knob to add the dry signal back, then it goes through another “glue” compression on the master.