I can't believe that you've never listened to this before, I think you have but just forgotten
Well since you asked me why it's a classic. Basically there's not too much to it. The strength of the album was its ability to blend metal muscle to melodic hooks, it had been done before but never as well and polished as Judas Priest did it here and the album effectively created the 'pop metal' term. It's largely thanks to this album that metal became a huge commercial draw in the 1980s and opened the door so much for the genre. It's not the band's best album but certainly their most important and a classic in every way.