Circumcision is indeed unneccesary, but is common in the US mostly from religious beliefs and lingering medical misinformation. Doctors used to blame all sorts of shit on the foreskin. It is far less common elsewhere in the world. 80% or so of men are uncut. Removing the foreskin is a surgical procedure that has negative effects, as it removes a part of the penis that has a specific function. Sensitivity is reduced, how much so is debatable. The glans (head) is desensitized somewhat, not having the protection of foreskin, but it still functions as needed.
Some women may not prefer one way or another, but the intact penis tends to have more skin to slide in and out of, reducing friction. The same benefit applies to masturbation, giving the penis a sleeve of sorts and reducing the need for artificial lubrication. Overall, being uncut has a couple slight benefits but nothing too considerable.
I just realized I know (or maybe just think) a lot about penises :/