Water goes to toilet water supply valve then goes up to the Vacuum Breaker then down into toilet when you flush
Hole in top of vacuum breaker closes off from water pressure
Then after flushing and toilet valve closes the vacuum breaker hole opens and allows water to drain down into bowl...that is where the water in bowl comes from
Water in bowl is a vapor barrier......stops black tank odors from coming backup thru toilet bowl seal
Water in bowl keeps bowl seal from drying out...leaking
Vacuum breaker is there by Design....replace it.