Compression vs Volume Automation

Lately I've been trying to really learn the mixing board aside from just "add a hint of reverb", and I'm focusing on Compression. I've learned how to link mixer inserts and run similar instruments through one insert to compress them together, to try to give a more "unified" sound to that group of instruments.

Up to this point, I've been making a volume automation for every instrument, and adjusting them through the entire song. Sometimes a slow, gradual increase in volume, a sharp curve drop, the automation is very useful in its hinge points.

By automating the volume for all the instruments, am I doing what a simple compressor would do, just doing it the long way? Or could I leave my volume automations, then add a compressor to groups of instruments and let it do further tweaks?

Thanks!