The benches are on the roof of the caboose for the same reason that there are benches in the cupola of the caboose from which Mr. Delano snapped this picture. They are there to provide a place for the rear trainman to ride where he can see the train ahead to inspect for signs of problems: derailment, shifted loads, smoke, etc. Also, to pass signals to the engineer (remember, no radios at this time in RR service).

Though most cabooses had a rooftop cupola, or side view bay windows, for protection from the elements, in the rush to press cars into service during the war some roads didn't bother to enclose the seats.

When or why would some need the bench seats on top of the caboose?

Looks like one of the wartime emergency cabooses the Santa Fe converted from wooden boxcars. Here's a little closer look at one:

The train we are "riding" is on the siding, and most likely not moving here. The man down the track, and probably another on the other side, is watching the moving freight on the left for any signs of equipment dragging or hot bearing journals. When the train passes, and they get clear, they will return to the caboose and proceed.