It helps distribute the clamping force of the lug nut more evenly about the wheel, and or reinforces the center of the wheel.

Look in this catalog.
https://www.accuridecorp.com/files/2012/10/Accuride-Wheels-Product-Catalog-Summer-2011.pdf
Page 27 shows and talks a little about clamp plates and they're required with some wheels.
See that GM part number 472536 is for a more modern clamp plate that uses 90 degree bevel nuts, and I assume 9/16" studs.

Original clamp plates from the AD 1 tons use flat faced 1/2" nuts.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-