The bulk of transformers are three phase transformers. If you have a load that is balanced (equal across all three phases) then you can use a transformer with no ground and save yourself a wire. If you are supplying three distinct different loads, then you need to proved a path for the difference between the three loads to flow through. That path is the neutral path. If you supply an unbalanced load without a neutral path, then when one phase draws power it changes the power available to the other two phases.

The best way to ground a power system is to obtain the system neutral through a generator or transformer with a wye-connected winding. However, a system neutral may not be available, particularly in many older low voltage systems and a significant number of existing medium voltage systems. To avoid a high cost of replacing a source transformer, an existing delta-connected system can be grounded using a star/ open delta or star/ delta transformer. During a line-to-ground fault condition, the zero sequence currents can flow into the ground at the point of the fault, and back through the neutral of the grounding transformer, hence providing facility to monitor & limit ground fault current. However Neutral Grounding Resistors are used for system grounding through Neutral Grounding Transformer & generators. A neutral grounding resistor limits the fault current, which is sufficient enough to operate protective relays, yet prevent unwanted fault damage. http://www.aei-co.com/products/transformer.html http://vawww.control.com/1026227249/index_html http://www.mathworks.com/access/helpdesk/help/toolbox/physmod/powersys/index.html?/access/helpdesk/help/toolbox/physmod/powersys/ref/groundingtransformer.html&http://www.search.com/search?q=neutral+grounding+transformer&nav=2.10.4.10 They are also called Zig-Zag transformers.

all electrical systems should have a link to ground.otherwise there will be severe ground insulation stress on transients. a neutral grounding transformer links the power system neutral to ground.

Here's one, it's self explanatory: http://media.techeblog.com/images/transformer_la_1.jpg

Vs prasad has given very good answer but one more thing i want to add that what is work of resistor with ngt if an alternator goes to single line to ground fault. In this case power output become very low and speed of alternatoe become more due to input shaft power and alternator can go out of step.Here resistor consume the power and give braking action.Hence speed of alternator may be controlled.Again why we need to minimise the zero sequence current?And answer is to minimise the interference with communication line.