Many living beings such as birds, fish, insects, and land animals perform certain operations such as moving, hunting, and foraging in groups. They stay and hunt in groups, because it makes them stronger and safer from predators than pursuing goals individually. So, let's say you want a group of birds flocking, swarming around in the sky; it'll cost too much time and effort for animators to design the movement and animations of each bird. But if we apply some simple rules for each bird to follow, we can achieve emergent intelligence of the whole group with complex, global behavior.
Similarly, crowds of humans, be it on foot or vehicles, can be modeled by representing the entire crowd as an entity rather than trying to model each individual as its own agent. Each individual in the group only really needs to know where the group is heading and what their nearest neighbor is up to in order to function as part of the system.