To get assigned to protect a president? Yes, that’s a pretty tough selection process. The Secret Service has other responsibilities, though. Presidential protection details are just one possible assignment. It just happens to be the most high profile and prestigious assignment. But they were actually chartered as a law-enforcement/intelligence branch of the Department of the Treasury, so they also investigate a range of financial crimes, including (but not limited to) forgery, counterfeiting, wire fraud, etc. At the time that it was decided the president needed constant protection (after McKinley was assassinated in 1901), the FBI didn’t exist yet, or else they might have gotten the job.