It's the best card to play in the game of Uno.
The player using this card calls a new color, and the next player must draw four cards and loses a turn. That can be a real game changer. Especially if the next player has called "Uno" and is close to a win. A wild card is an unexpected element. Even if your opponent has played his hand well and made all the right moves, a wild card can throw everything off. It's completely unpredictable.
And so it is with life... No matter how carefully you plan your life, a wild card can always come along and change your course. How you play it is up to you.