When you REALLY have to horse 6"-7" bluegills out of the lilly pads and brush piles, maybe? 😅
Yeah, saltwater is what it would be for. Like @Ham said...sharks, jumbo tarpon, alligator gar, goliath grouper, billfish, big mahi maybe...tuna, maybe big reds. How about for jumbo Blue Cats and Flatheads?
Most likely I wouldn't need a 12 wt...I'd likely be fishing with a guide for most of those saltwater species, and hopefully could use their gear.
I recently bought a new 9wt... mainly for Striped Bass, but might put that to use on some of these other species too.