Dogs roll in poop and their own poop sometimes to mask their scent which is an instinct behavior which goes back to the wild animal days when dogs were mostly wild animals.
The dogs would roll in the poop to mask their scent before they would sneak up on other animals which were there prey.
If you go out and observe wolves in the wild you'll sometimes notice if you watch the wolves closely that the wolf will roll in other poop and even carcasses of dead animals.
They do that to mask their scent before they sneak up on their prey so that the prey won't smell them coming.
You can stop the behavior by telling your dog firmly that they should not do that or spray them with a water bottle if they decide to roll in the poop.