I doubt an alien species would travel all the way just to wipe out life on Earth. A species could just send a Berserker probe to do their genocide.

An alien species can give a message that would seem threatening. For example, in the 1951 movie The Day the Earth Stood Still it shows electrical power around the world being shut off for a half hour. It shows multiple scenes of people in different countries astonished. Then it shows a meeting of U.S. government officials talking about the reports coming in. The average person knew an alien just turned off their power and that scared them. The government had a wider scope and knew that power was still working in situations where lives depended on it working like hospitals and planes in the air. It's scary for someone to turn all the power off. It's terrifying when someone has the control to pick and chose what gets electricity and what doesn't.

The message was given to draw attention to and demonstrate the seriousness of Earth's new role in the galaxy since Earth had achieved the rocket technology and atomic weapons to become a potential threat to other species.