According to this vintage box of Dr. Young’s Self-Retaining Rectal Dilators, they’re perfect for the treatment of constipation and hemorrhoids.
I like how the description by Cowan’s Auctions is a bit more honest:
...however they were sometimes used for other purposes.
These puppies were sold in drugstores across the country for $2.50 per set. Today they’d be sold in a sex shoppe for $250.