Why did the Shang rely on strong armies to maintain their rule?

First, the country governs other small states with their own army, so the country needs to build its own army to contend agaisnt with other states.
Second, the emperor in the early time of the dynasty liked invading and conquering other states, so an army is necessary.
Third, Shang Dynasty is a country that use violent revoluation to build the country, so it needed an army to be on guard of any revolution inside the country.