For the other, in penetration it both acts as liquid-less lubrication by allowing the penis to move freely inside it even if the vagina isn't very wet; and it also increases the range of sensations by feeling different when the movement is with the glans still inside the foreskin, and when it's deeper in and outside the foreskin. The "pseudo lube" aspect is also appreciated by women in my experience.
E: I see some people down-voted this and i am earnestly puzzled. I believe i have described only physical facts. Is this post too "frank" for a public forum, or did i get something wrong?