The thing about tractor implements is you need to be able to dedicate a tractor to them.
However if you already have a tractor you save the cost of buying a separate motor and paying to maintain it.
That does not sound like much if you are only thinking of one generator.
If you are thinking of several implements then the costs start to add up. They soon add up to where it is cheaper to keep two or more tractors than buy a bunch of different motors.
So with one tractor your PTO can run a mower, a bailer, a snow blower, a grain auger, a generator, a bail elevator, a water pump and a host of other items. The only restriction is you can't run more of them at one time than your tractors will power.

A short note about PTO.
You can run shaft, belt, air or hydraulic PTO on most tractors now. Some tractors are equipped with fairly large onboard generators to power electrically driven implements.