For me, growing up in the early 70s, that's where all the new, popular music was first heard.
AM radio was KING.
1050 CHUM in Toronto, 1290 CJBK in London, and several others brought me the sounds of Lighthouse, Brownsville Station, Paperlace, April Wine, David Essex, T-Rex, and a lot more. I don't think I even knew about FM radio until The Eagles invented in it the late 1970s.
By the time I started high school, I was listening to our city's college FM station, CIXX FM 106.9. They played all the "alt" stuff, or what was to risque for other stations at the time. It's the first time I heard stuff from the Jim Carroll, The Jam, Iggy Pop, and even Frank Zappa among others.
I'd have to say that both AM and FM radio played an enormous part in shaping my taste in music.