A few advantages:
1. Able to get a greater fire flow to any one point (hydrant) since water can flow from more than 1 direction, thus reduced head loss in the system.
2. If a section of line is shut down due to a break or other reason, likely that fewer customers will be out of service.
3. Less (or no)flow at the end of a dead end line causes stagnant water, which could cause it to pick up contaminants from the pipe or lining. Or in cold regions more susceptible to freezing.
