the automatic transfer switch(ATS) is simply a safety mechanism... it keeps your breaker box from having TWO incoming power sources at the same time, and backfeeding 120v from one to the other.
as for running the Generator when plugged into shore power, many times it's for nothing more than exercising the generator when in storage, but without having to unplug from shore power, etc.
Also, if you lost shore power, such as at a campground, and had to use your Generator, you wouldn't want the shore power restored without a ATS, since you could have problems with two different power sources.
The ATS 'defaults' always to Generator power, if two sources are present.
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
110,000+mi since '14 - US, Alaska, Canada (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )b