The best description for the purpose of a stall strip is to modify or "tune" the stall characteristics of a plane.

On the only aircraft production that I worked on, they were initially taped to the wing with duct tape, and for the few production test flights on each plane the pilots would stall them and report the results. The positions of the strips were changed to achieve a straight-ahead stall break. Once the thing was trued up, the strips were permanently mounted. I was surprised that a single location could not be determined for the design. Each plane had custom installations for the stall strips.

Sometimes they are used to stall one part of the wing before the other. Sometimes they are used to create a different feel for the pilot leading up to the stall. If the stall is unusually sudden, strips can cause buffeting that can be felt or heard before the stall occurs.