Why a Auto Transformer is used in a grid?

auto-transformers, where they can be used (depends on the transformation ratio), are cheaper than "regular" transformers. 

Other than that, they operate exactly like a conventional transformer. Transformers do not care about the direction of the power flow, they can always operate with MW flow from HV to LV or vice-versa. 

Tap changers are used to help control voltage, particularly if you have on-load tap changers. These devices are usually installed at the HV side of the transformer (so they operate at lower current levels). 

The direction of the power flow depends on the system conditions. I don't know if this particular generator is connecting a 220 kV "system" network to a somewhat radial ("sub-transmission") 132 kV serving loads, in which case MW will flow from 220 kV to 132 kV. You could also have lots of generation connected to the 132 kV system, in which case MW could flow from the 132 kV to the 220 kV. But I can tell this: the transformer doesn't care about it, it will work properly (as long as within its capability, obviously) either way.