The wiring of the generator output is fed into the main circuit breaker. It can also be modified in such a way that the generator only powers certain important circuits because it is likely that the generator may not be able to support the entire main circuit’s requirement.
The ATS is also a safety feature which ensures there is no “reverse supply” where the generator supplies electricity into the grid. This may be fatal to electricians working on repairing the grid. Therefore the ATS option is essential for standby generators.