Each level of these optional dungeons has a number of chests in it. These are generally single-use items of varying utility -- Potions and other shop-available items aren't uncommon, Red Fangs and spellcasting items show up from time to time, and every once in a great while you get something nice like an X-Potion or Elixir.
Here's the thing. The game has an internal list of what treasures can show up on what dungeon levels, and most dungeon levels
cannot spawn that many chests. If you want actual equipment, or the better items from the chests, you have to run these dungeons several times each. Earthgift Shrine is only five floors, but...
This one, the Lifespring Grotto, is twenty.
The decor here usually takes after the Chaos Shrine or the Sunken Shrine. It's a shriney place to be.
Since you're expected to run these dungeons dozens of times (each...), there are some new monsters to be found. But since they're all optional dungeons and the developers didn't want to guarantee you'd be overleveled from doing them, they give utter shit for experience. These Death Elementals, for instance, give about seven hundred EXP each. Compare this to the monsters in the Sky Fortress, which give two or three
thousand each. The rewards from the optional dungeons come in chests and from fighting the bosses, not in levels or gold.
There are non-combat levels here, too. This one is a library where the scholars wander around and get in your way. It's time-consuming and tedious, but not more so than fighting monsters for almost no reward every few steps.
On the fifth level of the Grotto is a mermaid village. All the mermaids talk about is a man with a halberd running around.
You have to track down the correct mermaid to lead you to said man.
????: Now I can finally go home! Hey! Where did YOU come from? Oh, you mean you were looking for the Excalibur, too?
Uh, no. Got one right here.
????: How rude of me. I mean, the four of you came all this way... Of course I'll let you have the sword.
...
I'd say you've been hitting the oxyale a bit hard if you believed THAT!
Surprise, it's Gilgamesh, the recurring villain who wanders from Final Fantasy game to game trying to get back to his own world and collect rare weapons.
If he has any tricks, I've forgotten what they are.
And here's the other problem with the optional dungeons: even once you fight a boss and earn its prize... well, the game already had endgame gear without all this rigmarole. If you find a shiny new helmet, is its tiny stat boosts better than "Half damage from most attacks"? If you get a new gauntlet or ring, is it truly better than "Immune to the death effects so many monsters and bosses are fond of this late in the game"?
No. You keep your Ribbons and Protect Rings on, and the new equippable gear gets thrown in the same pile as your Blue Fangs and Lunar Curtains and Vampire Fangs and stuff.