I've noticed a few people do this but I'm not sure why. As far as I know does it moisturise your skin? Or is there another reason to adding it?
Glycerin (or Glycerol) does work as a moisturizing and skin softening agent, so I would assume that would be one reason. I'm not sure how or if it affects the lathering properties of the soap though.
Moisturizes the skin, makes the lather slicker, and imo also prevents from drying on the face too fast.
I started with Musgo Real, which was drying my face too much, after I started adding glycerin to my super lather I have no problems with dryness with this cream.