The tradeoff with Bonfire trashing cards in play, vs. cards in hand is that while it can't get rid of the dead weights like Estates* and Curses, it does let you play Treasures and actions for their effects, and then trash them. So a "get your cake and eat it too" syndrome.
As an Event, it's always available. It won't run out, nor can it be embargoed, nor Contrabanded, nor blocked via Delusion
It's a way to "self trash" cards that don't have that feature built in. You're done with a card that was good early on, but not really needed later on (e.g. Sea Hag after all Curses are out, or Talisman/Quarry when you're done with <$4 cards and action cards respectively)
If you have the "penalty Landmarks" in play...
Wolf Den = Getting rid of a singleton or 2 gets you 3 or 6pts back
Wall = 1 to 2pts back
Bandit Fort = 2 or 4pts back for trashed Silvers and Gold
If you have multiple buys, then even more points back. In fact, if you're down to Provinces, and there are no other ways to get pts, then Bonfire lets you up your score in this regard.
Also nice if Tomb's in play!
* Inheritance was brought up, and yeah, same thing... if you took the turn to buy and put a card underneath Inh. token, then chances are, you'd like to keep those cards functioning that way.