Why is identification of the vector group of transformer important?

Vector group means it defines the primary & secondary side connection type of the transformer.For example DY says D for the delta connections (windings connected between two phases) & Y for the star connections (windings connected between two phases). That's why identification of the vector group of transformer is important.

Vector groups specify whether two or more three-phase Transformers can be paralleled with each other. In order to do so, their angular displacements must be the same. Transformer connections listed in the Vector Group tables, categorise transformer connections in terms of their angular displacements.