You'll get to fill out a boring customs declaration forum. No carrying food into the country or smuggling in plants/animals/pets, no carrying over $5K of cash or equivalent, etc. If you have nothing to declare sometimes you didn't need to fill out the form. I never understood that in the past, perhaps it had something to do with no checked luggage (or no luggage at all) equals no declaration form.

In the old days they were lazy and handed out Visa Waiver forms to everyone on the plane. Perhaps they've stopped doing that. The Visa Waiver form is the famous "are you a terrorist?" Q+A forum. You don't have to fill that out if you're a citizen because entering the USA as a citizen never requires a visa. Theoretically if you have a visa (are a citizen of a non-visa waiver country) someone already signed for you and investigated such that you're not a terrorist and filling out a visa waiver would be pretty pointless if you require and have a visa.

Immigration officials seem to delight in pulling peoples chains, and you have nothing more interesting to do while waiting in line, so local variation and variation over time is highly likely.