Step up or generator transformers increase the produced voltage of power generators in great power plants. Therefore it seems the unique function of these transformers is the increasing of generator voltage equal to voltage of national grid.
But as you know, in small and individual power systems (e.g. industrial power plant and related power distribution system) the generator transformers are used as the electromagnetic interface stage between generator and power distribution system. In this mentioned system the transmission and distribution voltage is medium voltage, therefore output of generator may be used directly in distribution system.
How can you explain the essential function of generator transformer in small power systems?