That's all they had to keep track of where your plane was from the air. Radio beacons came later.

Read about them in Wikipedia:

https://en.wikipedia.org/wiki/Airway_beacon

I was born and grew up in the Midwest and we still had towers with rotating beacons to guide aircraft in flight. Remember seeing them in many locations as we drove around the countryside. I think there was an extensive article somewhere on the WWW that told the history of the towers and beacons between Las Vegas and Los Angeles.

Here's one that still exists in Minnesota: