A loop course starts and finishes at the same point by running one continuous circuit, or the same shorter circuit repeated multiple times. It's common in city marathons because it keeps road closures contained to a single area. Multi-loop formats also make it easy to place aid stations and cheer spots that runners pass several times.
It's a route that forms a closed circuit back to the start point, either run once for the full distance or repeated several times. Both single-loop and multi-loop marathons fall under this category.
A single-loop course covers the whole distance in one circuit. A multi-loop course repeats a shorter circuit, for example running a 10.5 km loop four times to complete a marathon.
They make spectating and aid stations easier since you pass the same points repeatedly. Some runners find repeated loops monotonous, while others like knowing exactly what's coming next.
Yes, especially multi-loop formats, since you get familiar with each climb and aid station after the first lap. That predictability helps with even pacing later in the race.
They let organizers close a compact, manageable area of roads rather than a long corridor across the city. It also concentrates crowd support, which many runners actually prefer.