Urza is a singular win condition for the decks that can play him, capable of stabilizing a players life total, drawing additional cards, removing opposing cards under opponents control, creating creatures, and even generating virtual mana by making additional artifacts cheaper to play
The Jumpstart format has also been unbound from its paper roots where the original would simply give you a random selection of themed packs to pick from, WotC tells me it can better collate Historic Horizons packs to offer a greater variety of options and synergies each time, or even keep track of which packs youve picked previously to provide fresh options on your next run
For collectors, though, its value isnt tied to rarity
A Yawgmoth's Will on a permanent is nasty
And that I can use this Teddy to give players cards when they dont want them to fuel forced discard