To my knowledge all the nomad empires ever operated on a kind of "snowballing" dynamic - conquer a tribe, subjugate it, add it to your forces, conquer, subjugate and recruit the next one etc. etc. Usually they ran out of momentum sooner or later, but the Mongols managed to keep going more or less until they ran out of new nomads to forcibly recruit...

Naturally whatever khanate/empire/whatever said tribes used to have were rendered defunct in the process, which is probably what the sources usually mean by "destruction".