The Foreskin:
The outer foreskin is a continuation of shaft skin. It protects the glans from becoming calloused due to day-to-day abrasion on clothing.
The inner foreskin is densely innervated and has a continuation of the specialized membrane on the glans. This membrane keeps the glans moist and sensitive.
Combined, the inner and outer foreskin is a very large amount of tissue (estimates are usually 5 - 15 in2). It makes the shaft a mobile sheath that rolls back and forth over the glans, which makes sex and masturbation easier and more fun.
The Frenulum: A highly eroginous band that connects the glans to the shaft. Considered to be the most sensitive part of the penis to fine touch. Often partially or completely removed by circumcision.
Dartos smooth muscle: A highly elastic smooth muscle beneath the skin which causes the foreskin to be extremely stretchy. It tapers and hugs the glans, especially when a person is active or exposed to temperature extremes.
Glans sensitivity: At birth, the foreskin and glans are adhered together. The act of cutting a boy involves using a metal rod to tear apart the foreskin and glans. It can be extremely painful in the moment, leaves the glans raw and painful long after, and then leaves it calloused and unprotected for life.
Men who choose to get circumcised as adults have significant post-operation discomfort as the still sensitive glans rubs on clothing. After weeks to months, the glans callouses over similar to those of us cut at birth.
Autonomy: For those of us who were cut without consent (the vast majority of cut men), it is disfiguring to be given scars in place of attractive and fun parts of our sexual anatomy.