Many users of magic who had studied the offensive 
natures of magic, found that magic of a defensive nature
was just as useful. There isn't much that can stand up
to sheer power of offensive magic, except perhaps magic
itself. There are many spells which deflect, or help
one avoid the affects of spell cast upon them. This type
of spell requires a certain knowledge of magic and its
protective nature. Being skilled in this knowledge can
make a caster of protective magic much more capable.