It supposedly reduces sensation in the penis.
Meh depends on who you ask
The sexual effects of circumcision are the subject of much debate. The American Academy of Pediatrics points to a survey (self-report) finding circumcised adult men had less sexual dysfunction and more varied sexual practices, but also noted anecdotal reports that penile sensation and sexual satisfaction are decreased for circumcised males.
[25] A 2002 review by Boyle
et al. stated that "the genitally intact male has thousands of fine touch receptors and other highly erogenous nerve endings—many of which are lost to circumcision, with an inevitable reduction in sexual sensation experienced by circumcised males." They concluded, "intercourse is less satisfying for both partners when the man is circumcised".
[88] In January 2007, The American Academy of Family Physicians (AAFP) stated "The effect of circumcision on penile sensation or sexual satisfaction is unknown. Because the epithelium of a circumcised glans becomes cornified, and because some feel nerve over-stimulation leads to desensitization, many believe that the glans of a circumcised penis is less sensitive. [...] No valid evidence to date, however, supports the notion that being circumcised affects sexual sensation or satisfaction."
[75] Payne
et al. reported that direct measurement of penile sensation in the shaft and glans during
sexual arousal failed to support the hypothesised sensory differences associated with circumcision status.
[89] In a 2007 study, Sorrells
et al., using monofilament touch-test mapping, found that the foreskin contains the most sensitive parts of the penis, noting that these parts are lost to circumcision. They also found that "the glans of the circumcised penis is less sensitive to fine-touch than the glans of the uncircumcised penis."
[90] In a 2008 study, Krieger
et al. found that 'compared to before they were circumcised, 64.0% of circumcised men reported their penis was “much more sensitive,” and 54.5% rated their ease of reaching orgasm as “much more” at month 24'.
[91]
Reports detailing the effect of circumcision on
erectile dysfunction have been mixed. Studies have shown that circumcision can result in a statistically significant increase,
[92][93] or decrease,
[94][95] in erectile dysfunction among circumcised men, while other studies have shown little to no effect.
[96][97][98]
As far as we can tell with the research that has been done, circumcision doesn't have a net medical benefit.
Depends, it is definitly a positive in countries with high incidences of HIV and recommended as an effective countermeasure. Studies in developed countries are not very lucid, I don't think we can really judge what is the net result of circumcision.