The Spartans developed their militaristic society both as a means of controlling their massive slave population, and to maintain a societal status quo.

In the 8th century BCE, the Spartans began subjugating the local population around where the city of Sparta is (Southern region of the Peloponnesus). These enslaved peoples became the class of helots, that basically served as slaves and did the majority of the labor and farming in Spartan society. It's also important to note just how many helots there were, around a 10 to 1 ratio between helot and Spartan citizen. In the 7th century BCE, the helots revolted which forced Sparta into a very long and costly war which they eventually won.

Afterwards, Spartan policy was devoted to preventing any new uprisings. This is why the agoge was instituted, to insure all able bodied citizens could be trained to keep the helots in line and facilitate military expansion. Other social policies were placed to prevent any ideological differences in the society as well, travel outside the city of Sparta was heavily limited, currency was replaced with a system of iron rods, which look quite more austere than the silver coins of cities such as Athens, and any external showings of wealth were heavily frowned upon. And overall, tradition was a valued centerpiece of Spartan culture. All this was meant to prevent any outside ideas from entering Spartan thought, mainly through restriction of trade.

You might then ask why the Spartans needed this kind of system in contrast to maybe the city state of Athens. The contrasts are mainly the massive reliance on helot's. Most city states had a lesser ratio of slaves and free men as well working the farms while Sparta relied solely on helots. There also just weren't that many Spartans, this meant that Sparta had to utilize every single able bodied citizen. Which is often accredited to what led to Sparta's decline as a power in Greece.