DCAs don't route. Busses do. For a practical application of why this difference is importance, here are two examples.

Let's assign your drum channels to Bus1 (routing) and also DCA1 (level control.) Now since Busses can be processed, set a bus compressor how you want it and leave the threshold there.

Now let's say the drummer is playing a quieter song and you want to hit the compressor harder. You could do it with all the individual channels, but that is pain, and you are most likely going to change the relative levels. Instead, raise the level of DCA1, so it is increasing the channel levels equally going to the compressor, and hence compressing it more.

Conversely, if you like how the bus is being compressed, but the drums are just too quiet, raise the Bus level instead.

This is basically Pre/Post level, where DCA level is Pre compressor, and Bus is Post compressor.

Another advantage is that DCAs don't affect routing, so you can assign as many different channels being routed to different busses, and one DCA will control the level of each one individually without bussing them together.