Neutral Grounding Transformer

For short is about money. The insulation is very expensive when dealing with high and very high voltage. Grounding the neutral allows a reduction of the overvoltage (check further for over voltages in ungrounded networks as comparison).

Moreover, by neutral grounding you can also control the value for ground fault current.

Personnel and equipment safety. Without a neutral dangerous overvoltages will exist under certain system conditions.

Two possibilities:-

If you have a delta connected secondary on your supply transformer (3 wire only), then you have no neutral for use with single phase items. A simple zig-zag neutral grounding transformer can provide that neutral via an earth return process. The transformer maintains a fairly fixed neutral point and balanced return currents in all 3 phases when a single phase load is connected. Because the neutral is connected to earth there is no potential rise in the neutral. Standard fault detection equipment can be used for fault protection of the single phase equipment.

Other methods involve the use of a transformer with a star connected primary (star point to earth) and an open delta secondary and resistive load that offers a high impedance to +ve and -ve sequence currents but a low impedance to zero sequence/ground fault currents, thus allowing fault disconnection equipment to operate.

For personnel safety and for regulation!

Cost is a big driver. A grounding transformer can save investment dollars over many alternatives. Very little energy flows within grounding transformers under normal loads that are balanced. The size can be quite small if the ground currents are low and short circuit times are very short. To build the same capability within the transformer would cost far more. There are some advantages to some systems where there are delta-delta connections. There is a transformer connection which sort of provides a grounded delta caused a Heptner connection which is basically a Delta winding with an internal zig-zag winding connected in the case. This has some advantages on the overall impedance between windings, but it must be hand calculated for impedance, etc because it is so rare that no computer programs have been written, nor will they probably ever.

The main thing about grounding transformers, resistors or internal windings is that they must be designed with a system design and study and the duration and level of current allowed carefully accounted for with protective relays and breakers. The fact that this question was asked scares me since it shows no knowledge of electrical systems with enough energy to easily kill people. This is not a trivial subject and proper grounding of systems is extremely important.