By having a separate supply main and a separate return main, you can have independent heating control of each radiator or baseboard. You can remove a radiator or baseboard and have heat in the remaining heaters.
A one-pipe system can also give individual control of each radiator and with valves on the supply and the return of each, you can remove or replace radiators.
By running the tubing from one radiator to the next, you lose individual control of radiation, unless you use a bypass thermostastic control; although there would be no savings by doing so. A series loop would be controlled by a zone valve so all radiators in the loop would be on and off at the same time.