There are many spells that can do the reanimation trick, but Recurring Nightmare is a cut above them all, as it is the only one that can lather, rinse, and repeat for as long as you have three mana and a creature on the battlefield and in the graveyard
To put that in perspective, it's second only to Attack Forme Deoxys, also the second one ever to best Mewtwo's 154, leaving that one with the third highest at this point
Are you playing a commander because it is the best option for your deck, or because you like the specific card
Sideboard options Dredge sideboard is nothing special in the combo world, as its main use is to neutralise opponents hate cards
Black is "either an edgelord or was an edgelord"
Just like Inventory Management, this effectively prevents your opponents from interacting with this deadly card