It allows a player to put any card from their hand back into their deck to find a Pokmon SP
The absolute thrill of opening those Legends so that they could be inserted into Collector Boosters was a special, unforgettable experience, and you can read more about it in Mike Turian's Collecting Dominaria United article
Once per turn, during the End Phase, if this card was Summoned this way: Destroy this card, and if you do, take 1000 damage
As these boxes will become older and inevitably gain some value with time, there is an opportunity to grab them while they are under market rate and to get value out of them in the form of cards, alternate arts, playable cards and more.
Collector numbers are always done first in WUBRG order (white, blue, black, red, green) and then alphabetically in that color