Illusions have had many uses, mostly trickery and entertainment. One
more practical use is to cast a spell on a target which bends rays of
light around them, causing anyone who can see them, to see a slightly
distorted image. This causes any physical attacks to be somewhat
mis-aimed, since they are not based upon the true position of the target.
While it far from guarantees complete protection from physical attacks,
every little bit helps.