For as long as I've been aware of it, Radio 1 has followed a format of playing mainstream pop music during the day, with more specialised and esoteric programming in the evening. When I was a teenager, Peel's show was on at 10pm on weeknights, and his main focus was on championing new and undiscovered bands, across many genres. The Peel sessions were part of this show, where a band would come in to record a live performance of a few songs.
I imagine a lot of bands can credit their success to John Peel playing their records and inviting them to record a session.