well, they help with quads, you ass (gluteus maximus) and several other muscles, also some stabilizers.
Barbell on the back seems perfectly fine (
Barbell Lunge )
I used to do them with dumbbells (
Dumbbell Lunge ) i think that way it's a little safer.
But as you see on the webpage, it's an auxiliary exercise, meaning while squats for instance place greater in ntensity on leg/lower back muscles overall, the lunge may put greater relative intensity on a single muscle, in this case "A long lunge emphasizes the Gluteus Maximus; a short lunge emphasizes Quadriceps.".
So it would be an ok exercise to do alongside e.g. squats/deadlifts but shouldn't replace those.
Also i think they are ok for a beginner, as, while form is still important, chances of screwing up your back or something when not doing them perfectly are slighter. So especially when doing them explosively, risk of injury isn't as big for a beginner. but that's more of a guess