Many utilities use the tertiary winding in substations as a convenient source for station service power for the lights, heat, battery chargers and controls. This is secondary to its main purpose already mentioned.
The winding voltage is usually a medium voltage "standard" for the utility like 13.8 kV. The 480V or 240/120V station service power comes from a "standard" pole top or padmount distribution transformer connected to the delta tertiary winding.
Without the diagram we can't tell how your tertiary is connected. A couple of possibilities:
1. Phase B grounded internally, only A&C brought out through bushings- provides full 3 phase service to the station service transformers, using the ground lug for the grounded third leg. Two single phase transformers connected open delta is the most common (cheapest) method.
2. Single phase power only with one phase terminal not brought out. If only single phase power is needed, there is no need for the third bushing. I have not seen this approach, but it is possible.