I think it's important to students and to professors and teachers. If we were just dealing in an educational setting with any individual using publicly available work on the Internet, you could certainly make strong arguments that there's an implied licence to use that or that it might be fair dealing by that individual. However, in the educational setting there are many group uses of material.
Showing an Internet video in front of a class of students is essentially a public performance. It's a different set of rights. In making a work available on a course management site or a course website, you're making a copy available to each student in a class. It's when you get into these multiple uses that the idea of an implied licence or fair dealing becomes far more grey or hazy.
The reason this exception is important is to provide clarity, so that when students and professors use these works in various ways for educational purposes, they don't unintentionally run afoul of the law and unintentionally commit infringements. They need some safety in the law for the purpose of clarity.