That high level of flexibility is crucial in tempo decks that walk the line between aggro and control
How many sources of card advantage are good for your deck really differs on a case-by-case basis
It turns out that Magic players love casting spells for free, even if free in this case is pay seven mana for an artifact and cross your fingers you overcome the randomness to get what you want. The silver lining, of course, is whatever spell youre copying from your graveyard gets to stay there on the chance youll copy it again, unlike many cards of this ilk that exile the spell following the resolution of the copy trigger, so if you can find a way to finesse only one legal target for the Crucibles trigger (perhaps by ensuring the top two cards of your library are lands with something like Senseis Divining Top), then youll get to copy the same thing over and over
In the case of Avatar, the 15 most expensive cards would cost an eye-watering $4,230 to purchase, and that's just one copy each and without considering the most expensive card of the bunch, which goes for a staggering $5,300 on its own
Inti, Seneschal of the Sun : Allows you to Discover 3 when it comes into play, so exile cards from the top of your library until you hit a non-land card of mana value 3 or less, then cast it without paying its cost, or put it in your hand
Cards like Lilo, Making a Wish and Stitch, New Dog were just two of the many cheap threats packed into the amber/x aggro shell, with the purpose of getting characters into play as fast as possible, and then using some number of Lanterns or shifting using Stitch, New Dog to get Stitch, Rock Star into play ahead of curve
