I am not an expert so don't take this a the perfect answer but I see it like this.
If a fish picks up your bait and swims towards you (drop back bite) and it takes the ledger with it no line will be pulled through your buzzer so no indication. But with a bobbin the weight of the bobbin will pull line back through the alarm giving the audible indication. You can ofcourse watch the bobbin and see the drop back bite without an alarm. I use a homemade bobbin which weighs just7g and is just enough weight to show a drop back without much weight for a fish to feel if moving away.
Also with the foot or so of line being taken before line is taken from the reel there is less resistance to the fish than line being pulled of the lightest setting on my bait runner! So if you set the hook before the bobbin hits the butt ring the fish hopefully might not eject the bait?
Hope this helps.