Grixis Affinity is almost entirely artifacts, from the Bridge lands and Ichor Wellspring to Myr Enforcer and Refurbished Familiar themselves, making Dust to Dust a premium two-for-one that can take out key affinity enablers or a threat. Sprouting Renewal offers another flexible artifact/enchantment answer or a body to help race. Speed still matters since this is a fast affinity deck, so keep your low curve threats and cut only the slower selection spells. Watch out for Galvanic Blast hitting metalcraft for 4 damage once they have three artifacts out.