I always assumed that they existed to put down rebellions.
Dinosaur world? The giant steers on the Cowboy Planet? 
Seriously (?) though, in the (semi-official) Colonial Marines Technical Manual, it’s mentioned that the human-inhabited sections of space are divided into American, Anglo-Japanese, and Chinese controlled “arms”; and enemy groups called “Beebops” and “Watanabes,” as well as a quote from a captured soldier with a very Chinese name, are mentioned. So, they’d probably fight (or had fought) rebel groups and/or Chinese elements that threatened American colonies, and occasionally, dangerous alien fauna. A bit of an overview here.
Interestingly, the manual mentions that Chemical, Biological, and Nuclear weapons all seem to be part of the USCM tactics, and aren’t infrequently used…but that equiping androids with weapons or giving them uninhibited combat capabilities is prohibited by the Geneva Convention!