Anal glands.
In nature, dogs are "large and in-charge" at the top of their respective food chains. (Well except MAYBE if Lions are around)
They want EVERYBODY to know "Kilroy was here" and so every poop they deposit has a little drop of 'anal gland' scent on it.
These glands express whenever a dog poops because there are SIX sites of muscle attachment for eight muscles. And when the dog pinches the poop off - all eight of those muscles express.
Except if the dog is overweight - or not-exercised then those muscles are distracted away from each other by the dog's 'marbling' with fat - or the muscle tone all-over is lacking for lack of exercise.
Rarely there's no explanation. But that happens and we just don't know.
So sometimes a dog will get an 'over-full' anal gland and then the material inside 'dries' (inspissates) to make it smaller and thicker and then the anal gland abscesses.
They'll lick and scoot to artificially express the anal glands before that happens and they're USUALLY successful.
I can teach an owner how to express anal glands at home and there's a tutorial on my website at drjohnson.com

How to use Eggshells for Anal Glands:

Take a raw egg with shell ON, and put it in a blender - and pour that, shell and all in the bowl WITHOUT food - never put it ON their food. This is 'extra'. Twice a day until the poop is chalky and it's PROBABLE that on pooping this chalky stool can express a 'normal' gland. You could do two raw-egg feedings once a week back-to-back and call that 'painless anal gland expression' once a week, or 'great calcium supplementation, cause it is.
Some people give a shell-on-raw-egg once a day anyway. NEVER mixed in the food.
Can you cook it? Like, a scrambled egg but shell-on? Sure. But raw eggs don't bother dogs.
Many dogs won't suck up a raw egg with shell "pieces" in it. For example my dog Ajax needs it beat up to where the mouth-feel isn't crunchy. -drjohnson.com