This is important to remember because Commander decks are a fixed size
Casting a spell for 6 mana and getting a bonus 4- or 5-mana spell is awesome, but it means that you cant use as many cheap cards in your deck, lest you cascade from an 8-mana spell to a 1-mana spell
They can surprise an opponent for that last point of damage that you need for the win or help facilitate decks that like to see creatures enter the battlefield to trigger other effects
Example: Bennie Bracks, Zoologist is great as a card draw, but you need to create a token to do so, and not every deck creates tokens consistently
What set is Cutthroat Negotiator from
Since Commander's a singleton format to begin with (with exceptions like basics, Rat Colony, Shadowborn Apostle, and others), you can return all the permanents in your graveyard to the battlefield with this sorcery during the late game to finalize your win