To find out what the average listeners want to hear. That way they maximize potential audience, that leads to higher ad rates.
By & large: It works. It's successful. If it wasn't, radio owners wouldn't spend to do it.
Exactly. The reason the "Christmas Format" has worked for 30-something years is because it was birthed in the research era, and programmers have been able to ask the audience what they love, like and will tolerate and continue to fine-tune it so that it works again next year.

