As omnipotent as Spotify may seem, it still has its limits. As a streaming service it is now slowly but surely moving towards holding a monopoly in the music industry, yet becomes troublesome in the way it excludes particular music. Especially new artists will first need a mediator before being able to upload their music to Spotify.
On the flip side there is Spotify's drive to archive anything fitting its standards of procedure. This means there are tons of bulk CD compilations with mediocre renditions of classical music, copies of albums (deluxe editions), digitalisations of heretofore inappropriately labeled "world music", and so forth. Consequently, a lot of Spotify's content (an estimate of 20%) remains unplayed for various reasons.
Although self-destructive when successful, forgotify.com grands its visitors the (literally) unique possibility of playing those unheard songs. It is already worth a visit for the fun of it, with the possibility of actually finding some nice "new" tunes. Or it will just play creepy overly-religious children songs, who knows.