What is a return line in AC power transmission? Why is it needed?
Most often 3 phase is distributed in delta to use only 3 wires plus ground high wire to absorb lighting and transformed down to Y with neutral earthed. This is for safety and dielectric breakdown ratings of equipment w.r.t. ground. In a balanced system with no harmonic currents , neutral has zero current. 

However during line reclosures and normal operation, each phase may be unbalanced and thus Neutral carries the remainder of the phaser sum of 3 phases. The transformer output impedance often rated in per unit (p.u.)of rated V/A affects load regulation and Neutral voltage swing during power up transients and load transients,thus Neutral impedance is also important. For startup over/under voltage Line to Neutral on each phase is important to supply good quality power.

Since harmonics on a single phase are not shared by other phases and accumulate from each phase in the Neutral wire.  If the loads tend to be non-linear line to neutral, transformer stakeholders may choose to double ampacity of the Neutral wire to avoid conduction overheating. Transformers are rated for a limited imbalance near full load on any phase, in order to reduce short circuit currents and Neutral overload.