The glans of the penis is meant to be an internal organ, protected by the foreskin (just like the clitoris of the female is protected by the outer labia). When exposed as an outside organ after circumcision, its outer layer hardens and become leather-like in a process called keratinization. It reduces the sensitivity and feeling of the glans, thus greatly reducing pleasure during intercourse. Notice the surgical scars below the head of the penis above.
Compare this to the smooth, soft glans of the intact penis.