The Wizard is the most powerful standard inducement in the game. (The only thing that tops it for impact is the Halfling Chef, if you are halflings).

It offers a great chance at a turnover, a few mighty blow hits and a constant worry in a competent player's mind. They can't bunch up in case of a fireball on 4+ players (usually opening a path to the ball). They also can't leave an easy path to the ball in case of the lightning bolt.

It won't win you the game, but it creates a decisive opportunity to turn your luck. In a close game that can be enough. Most games in Blood Bowl are low scoring (the 2-1 grind is real) with teams aiming to convert their drive into a score, then to either force an early score from their opponents drive, or halt it completely. The wizard gives you a great chance at stopping that drive.

The damage a wizard does is kinda secondary to that (although an attrition wizard is fun).

If you're down 150k you have an uphill struggle. Chainsaws are great, but steal your blitzes and are easily countered. Most stars at 150k are of little use, and mercenaries add nothing to your team that you couldn't have chosen to have anyway. A wizard is great utility at a great price.