1. The foreskin itself is a hugely sensitive pleasure zone. It contains over 20,000 nerve endings and makes up over half of the penile skin. Uncircumcised men often report that the foreskin is the most pleasurable part of the penis. As well, when uncut men masturbate they don’t need lube - they can just stroke the foreskin up and down. The sliding motion during sex is also very pleasurable for both the guy and girl. The foreskin has tons of functions, basically.
2. The foreskin protects the rest of the penis. Imagine for a second, walking barefoot over gravel for a long period of time. What’s going to happen to your feet? They’re going to toughen and callous over. But what if you wear shoes instead of going barefoot? Your feet will stay sensitive. It’s the same with circumcision. Obviously men aren’t rubbing their penises on gravel (geez, I hope not) but penises do rub against clothing and bed sheets, etc. The glans (head) of a circumcised man’s penis is dried-out and keranitized from being exposed (the glans is supposed to be an internal organ when not erect)…and thus less sensitive. For an uncircumcised man, the glans is always protected and stays sensitive.