No matter what, militaries are always needed for protection in kingdoms, be that internal of external. It really doesn’t matter, what matters is that they have them and have an added feeling of safety as a result.
In Attack on Titan there are three branches: the Garrison, Military Police, and the Survey Corps.
The Garrison (emblems with roses) is the lowest ranked of the three. They mainly work inside the walls to ensure that they are patched when it’s needed. They’re also defense in case Titans breach which at the time of the first episode, hadn’t happened in about a century.
Then, we got Military Police (emblem with horse and blade), which isn’t much better. They have way more bragging rights because one of their main requirements is to protect the Kings from attacks and the like. Boy, do these people have issues not being crooked. There are so caught up in their titles that they don’t care much past them.
Finally we come to Survey Corps (emblem with wings). There isn’t as much bragging here because of their chargers or even their job description, no. You know why? Because their jobs are to survey the land, protect it, and survey the titan issue and learn about their nature. Because of this, they also die very frequently so they don’t have a lot of time for duplicity.
There are some very interesting characters in the Survey Corps.