Knowing different stencil making techniques is important because it gives the artist flexibility. it is useful for the purposes of problem solving. If one of your screens goes wrong knowledge of different methods of doing or fixing something can be the difference between a failed project and a successful one.
For instance, if part of your screen was removed in a place were you wanted no ink to show you could use screen painting fluid. You could also use it to quickly create a new screen/ layer for your work.
Each method also has a different look. Some stencils create a more painterly effect. Others appear to be more clean-cut. By layer stencils made using different methods, the artist can create a piece that looks very different when compared with a similar piece using only one method. I think that in order to really bring out the full potential of screen printing it is important to look at other ways of doing things.