They allow you to open packs from nearly any set imaginable, from the latest releases to vintage collections that are nearly impossible to find in the wild
Finally, Emerakul, the Aeons Torn is about as iconic as it gets, so it wouldnt surprise me to see her show up again now
They are able to "de-Synchro" themselves by returning themselves to the Extra Deck if an enemy card or effect is activated, and Special Summon two in-theme monsters from the Graveyard to apply their more powerful effects
They both go to the same zone, be it the hand if its bounced, the graveyard if it dies, or exile if its exiled.
Specific Pokmon, cards, rarities, foil treatments, and pulls are not guaranteed in any individual booster box