Setting up a colony across the Atlantic was always a long, expensive and above all dangerous process. Between the native tribes and pirates, not to mention the threat from competing colonial powers, it was always clear that the colonies would need to be able to defend themselves. However, maintaining garrisons was often more expensive than it was worth, particularly for the smaller colonies. As a result, it was often cheaper to simply hand swords and firearms to the colonists and let them handle it from there.
These militia groups are often the primary keepers of the peace in the countryside. Most farmers are members of one group or another, usually on a local basis. All like to pride themselves on the fact that they can be ready within a minutes notice, thus giving them the nickname 'The Minutemen'. Quality of fighting varies from group to group, but by in large they are fairly capable of defensive tactics and patrols, although they falter in the face of regular soldiers.
Different groups also have different political orientations in terms of not only allegiance, but also law. Some are friendly with the Confederacy and over native groups, others hold bitter grudges against them. Some are devoutly loyal to the Crown, others to the colonial assembly. And while some do their best to keep the peace, others are little better than jumped up gangs throwing their weight around.