When the commander is buffed with equipment and enchantments, the Steed can support and protect him in the attack
Players would run this card alongside Cascade effects, Cascade into Valki and then actually play Tibalt, cheating in a seven mana planeswalker that could draw cards, remove dangers, and threaten an ultimate that would give you all the options you could possibly need
What is working against this card is its cost
The right play is going to depend on the matchup (if they're a very fast deck it could be right to block), your hand (if your next play is a Ghoulraiser and your creature is instead a Walking Corpse, it is excellent to block), and your tribe along with your deck
Additionally, the meta has shifted in favor of other powerful decks like Buzzwole ex Celesteela, making it difficult for Darkness-type decks to thrive consistently