Depends. Milling is usually flavored as either literally destroying spell books (there's a reason your deck is referred to as your library), or invoking madness. Of course, there's also certain deck types that want to mill their own library, like the Threshold, Dredge, or Cascade mechanics, or others that want a full graveyard for various degenerate tricks. Or Laboratory Maniac, although that one also falls back into madness.
Magic is stupidly complicated, is what I'm saying.