Apparently collagen makes up 80% of your skins structure.
This is a good article that lays what collagen helps with, but essentially collagen is high in 3 amino acids that are lacking in other whole proteins, that are desirable for building collagen in your own body. It can supposedly help with elasticity of our skin, which considering that with hCG we often end up with excess skin from weight loss, anything to improve our skin seems desirable! That said, collagen powder do have significant calories when compared to the total number of calories we have to work with while on hCG. My collagen peptides powder has 40 calories per scoop. If you choose to try it on hCG, you may want to consider just having a 1/2 or 1/4 scoop so that the bulk of your calories is still going towards your actual food each day.
I also have an article on why you wouldn’t want to replace any of the protein servings with collagen as collagen is not even close to a complete protein.