Why would you use a three phase transformer in place of a single phase transformer?

the necesscity of two three phase Transformers operating in parallel are as follows...

• if the one transformer fails to give supply,then another transformer can be used in parallel and hance,continuity of supply can be maintained.
• one transformer can be easily take out from the supply for repair & maintanance.
• if the load on the substation increases beyond the rated values of transformer, then another transformer can be used to share rhe load of the substation.