A spectrum by definition is a scale between two extreme points. The use of the term spectrum has helped psychology a great deal. The approach that you either are or are not is a former and more rigid way of viewing things. This approach would often alienate individuals into thinking they are abnormal.
Whereas thinking of things in reference to a spectrum helps us to see that an individual does not need to be in either A or B but that they can be both and they can also be somewhere in between A and B.