Why did the Romans keep large armies on the borders?

Like any empire, it is supported by the armed forces of its country. So the Roman held on, relying on its numerous, in comparison with other countries, troops. The Roman Empire was simply forced to keep numerous cohorts of its troops, in all parts of its possessions, where it was necessary at a certain moment. A large army is the guarantee of the existence and functioning of the empire. Therefore, the Roman Empire was required to maintain a huge army, at the expense of the exploitation of other countries.