Barndoors, grids, flags, and snoots serve the purpose of cutting light spilloff from softboxes and light sources. Photographers & video productions use this to isolate the subject from the background without having to reposition the subject or using a dark background. 

Barndoors cut light from the edges, while grids & snoots simply direct the light in a straighter line; scrims & flags are larger solutions used on grip heads for bigger lighting effect control.