Back then, labels released the single so that you would go buy the whole album later on.
It's not like today, where people just buy the one song and skip getting the entire album. Singles were like the warmup, you would cop the single, and then when the album came out, you would go get the whole album too. That's why the music business was booming back then. We would all spend our bread. Nevermind has sold like 30 million albums worldwide.
For those true Nirvana fans on here, they know that the single dropped just a couple weeks before the album did. So the fact that it sold a million physical copies, is crazy. You only had to wait a couple weeks for the album to drop, and people still went out there and bought the single up in droves. That says a lot about how huge that record was. They couldn't even wait another two weeks for the actual album. But when the album dropped, those same people went out and copped that too. 30 million of them.