Various reasons. Religious adherence if muslim or jewish. To reduce incidence of urinary infections in first 6 - 12 months of life, important for babies born with urinary tract malformations. Subjects who need to self catheterize. To relieve phimosis (tight foreskin). To look like father or brother if they have been circumcised. To reduce incidence of hiv/aids. Mentally challenged males for hygienic reasons.
Treat tight foreskin. The main reason is for treating a very tight foreskin that cannot be retrated or a foreskin that retracts with pain. Medications can be used to treat this but usually fail. If left untreated, the tight foreskin can strangulate the penis (paraphimosis) and cause even more damage to penis including severe pain.