I think some religious beliefs or bans may have a certain contemporaneous reason for them, but the original reason might have been lost after a few generations. For example, maybe at the time of birth of Islam, pigs spread disease(say like swine flu) in that region and to make people stop growing and eating them, perhaps a ban was codified into the religion that persists to this day.

Cows being worshipped and protected from being killed for meat in Hinduism might have been a reaction to famines where cows could eat grass, weeds, plants that humans cannot eat and provide valuable protein to people in the form of milk for a long time vs. being instantly killed for meat. Which also explains why a significant fraction of Hindus are non-vegetarians but only eat chicken, fish and some goat/lamb, since those are not normal sources of milk.

On similar lines, I wonder if circumcision was thought to have some utility in those times before it turned into a religious edict. May be to reduce pleasure to decrease teen pregnancies which put a strain on families to raise a baby, or maybe slow down the spread of some widespread venereal disease.