The only thing better than doubling attack triggers is doubling instances of those same triggers
Where the World Championship decks showcased the strongest strategies and combos from that season, Theme decks and their successors (Starter packs, Guild Kits, Planeswalker Decks, etc.) usually revolve around a specific sets mechanics and act as an introductory product more than anything else
Its the exact kind of creature a control deck needs
Its built to help you format your custom cards like a real Magic: The Gathering card: mana cost, type line, rules text, power/toughness, and all the layout choices that keep your creation readable
Phyrexian Arena gives you an extra card each turn at the cost of 1 life, but Sheoldred immediately cancels that out by gaining you 2 life