The Romans brought the benefits of Roman civilization and culture to conquered territories,and thus assimilated their populations (at least,those that hadn't been killed or enslaved during the conquest) into the empire as a whole.
The purpose of this was to win conquered peoples over to the Roman way of life - a higher and more advanced standard of living would reconcile them to Roman rule.
Sometimes though there would be serious revolts against Roman rule - one in Britain and 2 in Judea were the most serious,so this policy didn't necessarily work everywhere all of the time.