This is a safety measure. You can't rely on ALL aircraft having functional (or installed) transponders. You must actively sense incoming craft because they don't always politely announce themselves.
At small unattended airfields, you can sometimes turn on the field lights by transmitting on a particular frequency (as if you were calling the non-existent tower in quick succession).
In the US, ADS-B is not required below 10,000 feet and when more than 30 miles away from the 30 largest commercial airports.