Seal skin has a unique structure making it stronger and more pliable and more waterproof compared to other leathers. The monks bought it for it’s high protective quality.
FYI you do know about foot wear by native Alaskans known as Mukluk (Pronounced like: Mud-luc's)
Mukluks or kamik are soft boots, traditionally made of reindeer (caribou) skin or sealskin, and worn by Indigenous Arctic peoples, including Inuit, Iñupiat, and Yup'ik.
Just something I learned from a native Alaskan friend I have. I'll leave out the part about soaking in urine to make the sealskin softer to work with, i.e. sewing.