To use a drone commercially, you have to have an FAA licensed drone pilot.
and appropriate spray licenses and a bunch of insurance as well.
They make a whole lot of sense spraying power line right of ways where aerial applicators can't go. A drone can make a pass where a spray vehicle can't go and where a guy with a backpack has to fight to get through the underbrush.
Another application is spraying the banks of lakes and ponds with properly labeled chemicals, no worries about snakes, slipping into the water, and way faster than a guy trying to spray and walk the banks.
I've been to seminars and seen videos of them being used on steep interstate slopes in states that have rocky, steep areas. They can be sprayed in minutes instead of putting men and equipment in danger on those slopes.