You are right,it is a jewish custom,christians and muslims also follow it as all three are followers of prophet Abraham(peace upon him). As for medical benefit,penile carcinoma and balanoposthitis are unknown in circumcised men.treatment for adult onset balanoposthitis and fourniers gangrene is circumcision and incidently circumcision is approved by WHO as a method of preventing HIV transmission and STD in central africa, source:Harrisons principles of internal medicine,16 ED. Bailey and Love,textbook of Surgery.
