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?
And yeah, i can entirely see how different people might not give a damn about any of this. Some might even benefit. Men who're not very sensitive would have more fun without the foreskin and the direct stimulation and might not even notice the loss of the nerves in the foreskin, men with very active glands might not care about the difference in lubrication, women whose glands are naturally more productive might not care which type of penis their partner has.
Nevertheless, the sexual function is changed. There are things a circumcised penis can do better. There's also things an uncircumcised one can do better. I'm not saying either of them is better than someone else's, just different. Changed.
Edit: can't reply, but I think you're being uncharitable by saying I'm overreacting to the word "mutilation" because it's technically correct by the dictionary definition. We don't describe people with earrings or tattoos as mutilated unless we intend to insult them or the practice.
That is not what mutilation means though. Like, literally not. Check the dictionary. You saying that just drives the point home that you're mapping a meaning onto it that is not intended by the other side.