Apparently the meibomian gland and the sebum it produces is the reason your eyes are water and air tight when they close.
If it was just two dry or moist pieces of skin pressing together, they'd need a lot more force to prevent water from getting in. But, with a thin layer of oil there, they sort of stick together to keep the water or air out and your tears in.
I actually think it's kind of great that a scientist who discovered a lot of things about tears ended up spending the rest of his life writing poetry. There's a kind of almost fictional quality to that.