WordNet sense 1 (kept in reserve especially for emergency use):

spare

WordNet sense 2 (not absolutely necessary):

inessential

WordNet sense 3 (unnecessary and unwarranted):

gratuitous, needless

WordNet sense 4 (more than is needed, desired, or required):

excess, redundant, supererogatory, superfluous, surplus

WordNet sense 5 (not necessary):

unneeded