I imagine that the primary purpose was to have dry peas for human and livestock food. Fresh peas were only available on a seasonal basis.
Some varieties of peas, like Alaska, are suitable for both fresh and dry use. I have tried growing these and other shorter-vine garden peas without support. It hasn't worked well in my garden.
Lucius_Junius wrote:. . . I've also heard of peas being grown in vast quantities in other places, and now I'm wondering how it was done. . .
Idaho, Montana, North Dakota, Oregon and Washington state are evidently where all 654,000 acres of US dry peas are grown. There are no supports for those vines. Harvesting is not very much different than the harvesting of cereal grains using combines.
We are each other's harvest; we are each other's business; we are each other's magnitude and bond. ~ Gwendolyn Brooks