When item cards are played, they stay in play and continue to grant benefits to other cards and the player
Second, it tutors a card into your graveyard, giving you full control that self-mill simply doesnt offer
They know that God-Eternal Oketra walks into their decks, that there is a Captain of the Watch who literally brings an army with him
This is a critical component of any Commander deck composition
There's a ton of ramp in the deck, more than I'd usually recommend, but if we look at the type of card draw we're working with -- lots of Clue generators like Tireless Tracker -- it makes more sense that we'll need more mana to crack them
Both have friendly to artifacts, but white is primary in that, and it shows