It's a keyword action, so it may indeed show up as an additional cost for casting a spell, but it may also be part of an enters effect, or the cost of an activated ability
Commander is an Eternal format and is typically played with four people
Most Rograkh/Kediss decks work in roughly the same way: play Rograkh on turn 1, buff it up on turn 2 with a Dragon's Mantle-adjacent effect, drop Kediss on turn 3, and then dump all your mana into buffing Rograkhs attacks
I do agree that I think Black Cat does do the job a bit better, but it's a niche enough archetype that a slightly more powerful effect might make that deck gain a little more traction
Proving this point, Murders at Karlov Manor proved that its possible to balance serialized releases
Its an interesting card to include and the cost is a bit high, but it will allow you to get back all of your best creatures by discarding a land