Rare cards often feature unique foil patterns, holographic effects, or special artwork
For example, if you control ten permanents, lose control of one, then planeswalk to The Golden City of Orazca, you won't have the city's blessing
Your creature only needs to attack and not even connect, and if you dont have anything to spend the extra mana on, you can instead grow the creature or dig for better cards
Getting a Vecna onto the battlefield tends to happen pretty quickly, which speaks to the strength of the focused decklist
Aside from that, the following cards mention oozes in their rules text, including a few token generators: Convert to Slime Gelatinous Genesis Corrupted Zendikon Gutter Grime March of the World Ooze Miming Slime Mystic Genesis Ooze Flux Ooze Garden Slime Against Humanity Slime Molding Gutter Grime is a strong enchantment for a typal build, but Slime Against Humanity is fun since it lets you break deckbuilding rules