Of either other dog, human, or other animal variety? Saw an article about dogs dieing of drug OD after eating addicts feces in a park.
What are they getting out of it? Undigested food?
I have three dogs and I can say for sure - it varies.
one of my dogs never eats poop except cat poop. neighborhood cats sometimes come over the fence and poop in my yard and every morning all my dogs run to that area like it’s an East Egg Hunt. 
one of my dogs eats poop every chance she gets. hers, yours, anyone’s. I tried everything and now I just scoop every time anyone goes and watch her like a hawk on walks. I believe she started out just liking it - the taste! and now it’s a habit. we did all that stuff where you add things to her food so her poop won’t taste good - no good.
my third dog doesn’t eat poop regularly - he ignores it mostly, but every winter a time or two he would want a Poopsicle. I’m not saying I put up with this, but that’s what he did - eat a frozen one.
and once? he was really mad at me and he went and got a big turd from behind the garage, came up near me so I could see him but not reach him - and he very clearly eat it AT Me. so, spite. I’m not kidding. (this was when dog #3 was still a puppy and I didn’t know she was a poop eater yet. so I was slack about cleaning it up)
short version? dogs have horrible tastes and there is no one reason they eat poop.
An argument I’ve heard, and I’m not sure how true it is or not, is that it is due to evolution because dogs are trying to cover up odors that would give them away to predators.
The argument was the dogs may have only eaten their puppies feces at first, so other animals would be less likely to know where they were hiding. But if that is true shouldn’t all animals do that? Plus what benefit is there of eating other animals feces? No idea.
dogs have fast throughput. this allows the eating of rotten shuff and shit without serious consequence. it also might leave still food value in shit. so if your nature is to gorge maybe rare food every chance you can, then go for it.
i’ve seen dogs shitting while eating rotten guts and then turn around and eat their shit.
Wow, did I ever LOL over that line, but I have no idea why I found that so funny. Humor defines character, I guess.:o
Cat feces, particularly, still has a lot of protein and such in it. Lotta good eatin’ still on there.
yep - my dog are SO enthusiastic about cat poop they want me to get them a cat just so they can lick her butt.
so far I have refused and, of course, I would defend the cat’s right to Not Put Up With That if she so chooses.
it IS funny and really amazing we love dogs so much when they can be so gross
And if it’s straight out of the litter box, they can pretend it’s Tootsie Rolls with sprinkles.
Are dogs the only animal that will eat their own regurgitation?
Cats do.
Not if there’s a dog in the house. Dog will shoulder the cat out of the way.
I can think of two reasons, in encumbered by research or actual knowledge on my part:
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Fat. Depending on what was originally eaten, there could be a relatively high amount of undigested fat, which dogs love. Me too, now that I think about it.
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Many vitamins and nutrients are generated by bacteria in the lower gut ( vitamin K, for instance, if memory serves me). However, the lower GI tract isn’t very effective at absorbing these nutrients, so you need a ‘second pass.’ This is one reason rabbits will eat their own poop.
Of course, I could just be full of shit.
Well, there was this one roommate of mine …
Off-topic, but something that happened many years ago. I pooped outside near some horses in a field and one of the horses ate my poop straightaway. Chickens will as well.