Wazir is one of the 9 players of chasing team. He is the one, who is allowed to move in any directions he seems beneficial, while chasing the defenders.
Except the Wazir, no active chaser is allowed to change his direction before reaching the free zone, while chasing the defenders.
The Wazir is considered a key player because he is free to move in all the directions, which makes it easy for him to tag the defenders quickly, and thus bagging much needed points for his team. Simply, he is a trump card for the team.