But if you have more than one Will available, then you can spend multiple points of Will to be able to cast more than one die to determine results. The dice would not be added together but rather you can pick the best result. This was important if you really needed to get off a critical spell and helped the greater magic users (those with a free Will each turn) to stand out above the rest. With a decent pool of 'natural' Will and the free Will from their Staff or other source they could cast two or even three dice a few times per game without much risk.
I mostly used this with Saruman to be sure his Sorcerous Blast went off well.