CardGroveKnows where every card is
Moxfield tells you what decks you have. ManaBox, how many cards. Neither tells you which binder holds the Sol Ring you need tonight.
Signing up needs an invite code.
Which box it's in
A location tree you name the way your shelves actually look: shelf, binder, page. Look a card up and it tells you where to go for it.
Proxies count
With their own art and as first-class objects, not a footnote. Two proxies of the same card with different art are two different things.
What the deck is missing
Paste the list and it tells you what you own, what is sitting in another built deck, what you have a proxy for, and what completing it would cost.