The dips in the cover become the lowest points in the pump's impeller cavity. These low points will collect any water that separates out of the oil, after shutdown and cool-off.
In colder areas, with freezing temps, these dips will keep frozen water (ice) from jamming the pump's impeller and causing a failure.
Many earlier factory pumps also have the "dipped" cover.
Aftermarket pumps come either way, dipped or flat covers.
DD

checked the slant six motor inventory that I have,, a: 1966, 1968, 1974, 1977, 1983, 1987 all have the the pump plate pockets, my mistake in thinking that the pockets were late Mopar changes. Looks like that the Dynagear aftermarket pumps and maybe others substitute in a flat pump plate.

Doc,,, Thanks for info on why the plate pockets exist.

for me,, I have a aftermarket dynagear with the flat plate. Think I will get a pane of glass, about 10x10 cover with 220 or better sanding material and re surface the 83 OE plate with the pockets, get it really flat to put on the aftermarket dynagear pump.

Good idea, I swap the "pocket covers" onto new pumps all the time.
DD