When they die, you reveal cards from your deck until you hit a creature, which you slap straight onto the battlefield
It provides the ability to grind out when you run out of cards, almost giving you an extra card draw each turn, but better, as it's a permanent of your choice from your graveyard, nulling any efforts by your opponent when using removal spells
Another cracking white card is Dawns Truce, the White Heroic Intervention
If the card in your graveyard is an illegal target as Emergency Weld tries to resolve, Emergency Weld is removed from the stack and you won't create a token
How do you achieve that
In the right context, its a central lock piece in a control deck that hinders the development and success of any archetype that emphasizes one and two-mana spells