Why does "Christian" Radio exist?

At first glance, one might assume christian radio exists puely to evangelize. But upon futher examination, one can find another more concrete motivation: money.

The recent market revenue report for the Dallas, Texas radio market offers a prime example. The report estimates that the "christian" formatted radio KLTY - owned by the Salem Media Group - generated more than $20,000,000 of advertising revenue in 2016.

While the airwaves are by law reserved for serving the public interest, the profit motive still pervades USA broadcasting; and often creates stange bedfellows in the process. So is using the airwaves to sell religion for profit really "in the public interest"?