Astigmatism is a relatively expensive card. It has high HP and has the ability to kill the monster in front of it when every time your turn begins, effectively making that slot useless to most of the opponent's cards until Astigmatism is destroyed. Also, if it couldn't kill a monster, it will add a Loox to your hand.
It's best saved for late game when wishing to destroy a powerful card, and can work well when given the Taunt stat via Protection (in Patience) or with the use of Glad Dummy (in general). It can also work well with Madjick to give you a glass cannon.