In some cases, it's literal card advantage (Decree of Pain)
Given your ability to dictate both the pace of the game and your own draws, you can judge whether a game is ripe for decking and then play miserly with your resources until youve absorbed every last drop from your opponents neck uh, deck
For Starving Revenant's enters-the-battlefield ability, you put cards into your graveyard for surveil (if any) before drawing cards
Summary Kindred decks are popular in Commander, rewarding both veterans and newcomers with synergistic strategies
Counting the cards in every players hand could easily leave this with 20 or more power
Not all Jaces are created equal though, and some of Jaces designs have been extremely niche, experimental, and sometimes just not good