Let's start with some examples:
Are Julie and Jennifer really both stimming?
Furthermore, the researcher in me is insisting on pointing out that when trying to better understand something, ANY distinction is important: if you group two opposite things together, findings from one negate the findings of the other, and there appears to be no result. It's like adding positive 5 and negative 5: the answer becomes zero. Minimizing error in research helps us to better understand experiences and needs, which in turn helps to improve our quality of life.
While stimming is usually harmless or even beneficial, sensory sensitivity costs autistic children and adults immeasurable energy every day. Recognizing when a repetitive behavior is an attempt to alleviate sensory discomfort (and, of course, correcting the discomfort) can completely change daily experience. Sometimes it's hard for autistics to recognize our own needs; it can be equally difficult for the parent of an autistic child to interpret these signals. Knowing the difference between a stim and an attempt to alleviate negative sensory input is an important step towards creating a positive environment, for yourself or for your child.