Why do people are antibiotics necessary for people with congenital heart disease before operations?

Depends on the kind: Of congenital heart disease and the kind of operation, but it is to try to prevent an infection in the heart inner lining, so called bacterial endocarditis.

Endocarditis: An infection of the inner layer of the heart can occur in patients with artificial tissue within the heart. We no longer recommend general antibiotic prophylaxis for all patients with congenital heart disease. According to current guidelines only patients with artificial tissue implanted or inserted via surgery should get prophylaxis.