In transmission lines, the neutral point is usually earthed at the supply end for protection and safety reasons.
Neutral is just another hot wire, but it is bonded to ground to improve system safety. This has the effect of keeping all hot wires within design voltage of ground, which is 120V in the case of 120/240V split-phase or 120/208V 3-phase “wye”.
Three-phase systems may or may not have a neutral wire.
A neutral wire allows the three-phase system to use a higher voltage while still supporting lower voltage single-phase appliances.
In high voltage distribution situations, it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).