With cards like Jeskas Will, Professional Face-Breaker, and Prosper, Tome-Bound seeing a ton of play in Commander, this card is in a prime position to have a big impact on the table every time that it hits the battlefield
it means you can get lots of extra mana if the creature generates mana, attack with abandon because youll be able to defend and, if your creatures have activated abilities, youll get to use them more

Common (36) 1 Brainstorm 1 Dark Ritual 1 Deny Reality 1 Commander's Sphere 1 Wayfarer's Bauble 1 Command Tower 1 Dismal Backwater 1 Evolving Wilds 1 Forgotten Cave 1 Path of Ancestry 1 Swiftwater Cliffs 1 Terramorphic Expanse 8 Mountain 8 Swamp 8 Island Uncommon (20) 1 Heralds of Tzeentch 1 Nurgle's Rot 1 Bloodcrusher of Khorne 1 Chaos Terminator Lord 1 Herald of Slaanesh 1 Noise Marine 1 Seeker of Slaanesh 1 The Lost and the Damned 1 Bile Blight 1 Bituminous Blast 1 Assault Suit 1 Sol Ring 1 Talisman of Creativity 1 Talisman of Dominance 1 Talisman of Indulgence 1 Worn Powerstone 1 Barren Moor 1 Crumbling Necropolis 1 Molten Slagheap 1 Temple of the False God Rare (41) 1 Lord of Change 1 Blight Grenade 1 Great Unclean One 1 Mandate of Abaddon 1 Mortarion, Daemon Primarch 1 Nurgle's Conscription 1 Plague Drone 1 Poxwalkers 1 Sloppity Bilepiper 1 Tallyman of Nurgle 1 Venomcrawler 1 Aspiring Champion 1 Bloodthirster 1 Dark Apostle 1 Keeper of Secrets 1 Khrn the Betrayer 1 Knight Rampager 1 Let the Galaxy Burn 1 Blood for the Blood God

First, it makes hasty creatures (like Arclight Phoenix) much less scary, since even though they have haste, they can't attack the turn they come into play
Some of the most expensive MTG promos that players have ever seen arent actually that old