It happened when her record company was urging her to write another hit in the 70s, after "Both Sides Now" and "Big Yellow Taxi" became big sellers. The head honchos wanted something else to keep her on the charts (and on the radio), so she wrote a song specifically to accomplish that. 

“I decided there were some ways to make a hit, increase the chances. DJs have to like it, so you put a long part at the beginning and the end so the DJs can talk over it."

The tune even mentions the medium - "You Turn Me On, I'm A Radio" - in the title, and it did, indeed, become a well played song in 1972. It only hit #25 on Billboard, but reached all the way to third spot on the RPM Chart in Canada. 

The song Joni Mitchell wrote after her label demanded a hit