Common vs Ground in DC Circuit
When talking about switches, "COM" is not at all the same as ground. It's the "common" input/output to the switch. The "NO" or normally open terminal is disconnected from COM when the switch is inactive, and connected when the switch is active. The "NC" or normally-closed terminal is connected to COM when the switch is inactive and disconnected when the switch is active.

In this diagram L1 is the NC terminal, L2 is the NO terminal, and COM is the common terminal:



The only reason to connect COM to ground is if you want to switch the other terminals to be connected or disconnected to ground.