Why the transformer is needed to be operated in parallel?

Two transformers are connected in parallel means that the two primary windings are connected to supply bus and the two secondary windings are connected to load bus-bars
NEED-:

• To supply a load in excess of the ratings of an existing transformer, Two or more transformers may be connected in parallel with the existing transformer. This is more economical connecting an extra small transformer in parallel instead of keeping an another large capacity transformer. The cost is also less for purchasing extra small rating transformer.
• Parallel operation of transformers provides more reliability i.e. even in the failure or out off service of one transformer half of the bus load can be driven using signal transformer in emergency cases.