> I don't know what is normal, but releasing singles or EPs before the full album seems like a common way to generate hype beforehand.
I think this arose in the radio-and-CD era of music. Maybe even earlier?
The radio stations that drove sales of new music wanted to play the latest releases. By releasing three singles before your 12-track album, you got more radio play, more shots at doing well in the sales charts, and hence raised your album sales.