TwoSix
"Diegetics", by L. Ron Gygax
Dream of the Blue Veil fills a particular niche; it allows the party to travel somewhere without knowing what that other place actually is. Plane Shift doesn't really matter if you don't know that Eberron or Toril actually exist. Dream of the Blue Veil lets the DM seed a magic item or an NPC into the game to explore other planes while still maintaining a sense of mystery. It's a plot device, just like putting a "magic portal" into your game, but comes with the shiny imprimatur of officialness for those who care about it.