The full answer
A restaurant is naturally interested in selling its food.
Independent certification adds another party whose responsibility is to verify the Halal claim.
Instead of asking consumers to investigate slaughterhouses and distributors themselves, a monitoring organization can perform that work and provide traceable certification.
Answers in the Halal Knowledge Center reflect the standards followed by Halal Monitoring Services (HMS), which operates as part of the Shariah Board of America. HMS certifies only what it checks.