I think I remember hearing that it was really useful against that Wall boss in IV? I really should replay all these FFs again.
It's useful against pretty much everything in IV. I made a habit of using it during my second playthrough and the difference in difficulty is very noticeable even with the limited augments I picked up
Slowing bosses down gives you time to recover if you're in trouble/underleveled. You're reliant on Rosa to keep the party healthy and her MP will run out eventually so switching to Pray + Cura from Cecil + Potions (+ Life's Anthem) is a viable strategy when the boss can't attack as often and Rosa is out of MP/ethers won't cut it
Even if the boss is one shotting your party members, you can still stand a chance by using Phoenix Downs/Life on dead party members and continually battering the boss until it dies (of course this doesn't vs bosses like Cagnazzo that can hit everyone)
You'll want to cast Bio on the boss as well because Sap is just a downright ridiculous status ailment
That's not to say FFIV is easy - Cagnazzo, Golbez and CPU got me a few times that first time I played the game
Honourable mentions to that ******* Dr. Lugae with Reversal Gas who makes it so healing spells damage you...this was before I decided to just use an elixir on him. Or maybe it was a Cure spell I can't remember