This is due to the following reasons:
1. Continuity of service
If a single large generator is used in the power plant, then in case of its breakdown, the whole plant will be shut down.
Generators run most efficiently when loaded to their rated capacity. Electric power costs less per kWh when the generator producing it is efficiently loaded.
3. Maintenance and repair
4. Increasing plant capacity
5. Non-availability of a single large unit
In many situations, a single unit of desired large capacity may not be available. In that case, a number of smaller units can be operated in parallel to meet the load requirement. Generally, a single large unit is more expensive.