As a little boy (he was born in 1911) during WW1, with Uncles away at the Front, my Dad went with his Grandpa to collect sphagnum moss from the moor. He said they collected three sacks full when he visited. The sacks of sphagnum then went to be processed to make field dressings for the soldiers.
Grandpa Reid had six sons, all in Highland regiments, all away at war, and he went every week to collect the moss. Only five of his sons came home. Two others were injured and they did use the field dressings. Both healed very well.
I know of sphagnum moss as the traditional packing for babies nappies, for women's sanitary wear, especially after childbirth, as well as for bandages.
It was just an easily available, generally at hand absorbant material. It's mildly antibiotic, it can lower the pH enough that it can help stop food spoiling, or wounds festering.
But it was just used much like we use cotton wool or paper.
Pick it, squeeze it a bit and pick it clean of debris. Lay it out to dry and then it'll keep for ages, ready for use.