The main reason is to blow protective fuses, to ensure that the fault current is sufficient for that purpose. However it also helps limit voltage excursions in 3 phase distribution.
Live to chassis ground is a common fault. Without neutral being bonded to earth, no significant current would flow to blow the fuse and disconnect the live.
Consider a 3 phase local distribution transformer, 240v phase to N, 415v between phases. If a live to ground fault grounded the red phase, then N would become 240v to ground, and blue and yellow phases would become 415v to ground, putting more stress on the insulation in all the other properties taking their single phase supply from the same transformer.