A list I found on the Internet seemed appropriate. Some of these items refer to straight sex, but most are applicable to sex with men or solo.
Source: www.cirp.org/pages/anat/ (Circumcision Information and Resource Pages) The list below list was compiled from various sources by George Hill.
The foreskin has twelve known functions. They are:
1. to cover and bond with the synechia so as to permit the development of the mucosal surface of the glans and inner foreskin.
2. to protect the infant's glans from feces and ammonia in diapers.
3. to protect the glans penis from friction and abrasion throughout life.
4. to keep the glans moisturized and soft with emollient oils.
5. to lubricate the glans.
6. to coat the glans with a waxy protective substance.
7. to provide sufficient skin to cover an erection by unfolding.
8. to provide an aid to masturbation and foreplay.
9. to serve as an aid to penetration.
10. to reduce friction and chafing during intercourse.
11. to serve as erogenous tissue because of its rich supply of erogenous receptors.
12. to contact and stimulate the G-spot of the female partner.
And, for #13, I’ll paraphrase from information on that page: Since more of the loose skin of the penis remains inside, the lubrication is not drawn out to evaporate to a great extent, which makes sex easier with less lube. (They said this in the context of vaginal sex, but it makes sense in general.)
And for the illustrations (not from that site):
BEFORE
AFTER