The majority of these flow control devices are either wind tunnel or flight test fixes (and the 1900 is a good example) for inadequacies (unfortunate or intended) in the original design.
They all do much the same sort of thing in that they provide a local and comparatively high energy airflow stream which interacts with the existing wing flow to improve deficiencies which are found to exist during the certification program.
Examples are to delay separation (shed vortex from a nacelle strake or LE VGs) to improve stall speeds or aileron control through to fences/notches/VGs to restrict spanwise flow.