It often leads to more creative deckbuilding decisions than might arise from apps where you have access to everything
If you can get some milling going (see above for what milling means!), you have a side plan of The Master perfectly enabling Reanimator strategies, getting to tap him to put the likes of Razaketh, the Foul Blooded or Hoarding Broodlord onto the battlefield
If you are relatively new, or are just ripping open booster packs for the first time, you may want to cover all of the rarities of the cards first
2/4 stats and Vigilance let it hold off a lot of the more aggressive creatures in Modern well
A tiny ding on the back of an Ancestral Recall can be worth a thousand dollars or more depending on the edition and condition of the card
Another popular card and commander for poison decks is Atraxa, Praetors' Voice because it allows you to proliferate on your every end step