The findings indicated four main reasons for repetitive behavior: enhancing the ability to function; reducing external stimuli and avoiding communication; coping with stress, distress, and excitement; and coping with social communication.

The findings suggest that repetitive behaviors play a functional role in people with HFASD and also serve as a way for these individuals to cope with their environment.

Source: Repetitive behaviors: Listening to the voice of people with high-functioning autism spectrum disorder – ScienceDirect

Functioning labels and medical model language aside, this study reinforces what autistic people have been saying about stimming.

For examples of popular stims, see this thread.

My contribution: