A Master's of special education degree trains you for the demanding and extremely rewarding task of teaching those children who are damaged in some way. It's perhaps politically incorrect to put it that way (which is why the "special needs" description) but this is effectively what it is. To do this job you'll need to have your regular Bachelor's degree and a teaching license: the reason for the Master's of special education degree is that over and above the general requirements of teaching there are a number of highly technical points that need to be learned.

There's many different types of special education. What makes it all so complex, and thus requires that Master's in special education, is that as a qualified special needs teacher you are responsible not only for teaching such children, you also need to be able to identify those who require such extra attention. Over and on top of that, each specific special need has its own teaching methods. For the mentally retarded or the severely autistic, it's a matter of trying to inculcate some basic life skills. For most others though (the hearing or visually impaired, etc.) there's no problems with intelligence or desire to learn: it's rather that regular teaching methods are ineffective. As each such group requires different solutions, special needs teachers are trained in one or other of the various special solutions. That's why the extra step of a Master's of special education degree is require, as there's a great deal more technical knowledge to acquire.