In order for a court to issue a warrant -- for someone's arrest, or to search or seize property -- there must be probable cause. Probable cause for arrest exists when facts and circumstances within the police officer's knowledge would lead a reasonable person to believe that the suspect has committed, is committing, or is about to commit a crime. There must be a valid reason for the person to be searched or seized.
This is included in the Amendment because without it, people could be unfairly searched/seized unfairly, based on the officer's hunch or suspicion.
For example, if someone were suspected of a crime, an officer could search/seize the person if they were acting violent or had something like blood on their clothing to make them suspicious. But they couldn’t search/seize the person just because they just look like a person to commit a crime.