City Water Systems

A water system is being added to my rural area. 18 or 24" pipe running along the side of the road in the neighborhood.
There are occasional 3" pipes with an elbow standing above ground. What are these for, flushing the new pipe when water begins to run?

Outlets from a water main are normally taken from the side at a 45 or 90 degree angle. Fire hydrants are normally 4 inch pipe and they are equipped w/ a long stem valve, to prevent freezing. Domestic supply is normally one inch. They could be for commercial buildings, or short extensions into streets (proposed streets). If they come directly off the top of the main I have no clue. This would be unusual because it tends to cause damage to the main if the branches are subject to stress.