Well with rock music...that was a genre that thrived on pleasant surprises, which can no longer happen in the era of digital downloads.

Rock music was born alongside the 45 RPM single. That's how kids in the 50s consumed pop music. You'd buy the record for the A side...but they often put something the label had less faith in on the B side, often something the artist was pulling for. And "The B side turned out to be the big hit!" is a tale told over and over again for three decades. From Rock Around The Clock to You Can't Always Get What You Want.

It mostly stopped in the 80s with the advent of CDs. CD singles were a thing; that's what mini-CDs were originally meant for, but in North America at least it wasn't really a thing. But with CD albums...A CD holds twice the audio that an LP does, so you start getting bonus and filler tracks, sometimes hidden. That's where a lot of "The artist was really pulling for this track" songs went. Billboard changed the rules to allow songs to chart even if they weren't released as physical "singles" around that time. Which is why Bon Jovi's story isn't "Livin On A Prayer was the B side of You Give Love A Bad Name" it was just track 3 of Slippery When Wet.

The rise of digital downloads/streaming killed this.