Circumcision is primarily performed for cultural or religious reasons.
Many think that foreskin is an unneccessary part of the penis. Many circumcisions are performed because a circumcised father often does not want to feel that he is different from his son.
It is often said that a circumcised penis is cleaner, or easier to keep clean, that an intact penis. Smegma (a natural substance composed of dead skin cells, normal flora, and secretions containing the natural antibacterial agent lysozyme) is more likely to accumulate when the foreskin is present.
Medical reasons for circumcision is 1. reduced risk of urinary tract infections (UTI) 2. reduced penile cancer 3. reduce risk of cervical cancer in partners 4. reduced risk of sexually transmitted disease (STD)
There are contradictory evidence in reduces UTI, but this is strongest reason of medical claims in favour of circumcision, because UTI cause serious condition and need antibiotics treatment. Baby boy in US about 1% have UTI but higher incidence for baby girl. There is evidence that the babies who are breastfed have a lower incidence of UTI.