The first function simply divides the image into black and white with colors above the set value being white and those below being black. You can then use the slider to move that line to enhance detail that may otherwise be obscured.
The second and third functions displays the image in either a grayscale or color so the artist can focus on the structure and design of the image.
The last 3 functions are Levels. They set either 3, 4, or up to 10 levels of color spread over black, gray and white pallets. The 3 and 4 levels have sliders to adjust where those levels are while the Levels function just evenly divides the image by the number of levels.
And of course, share your image in many ways.
Any artist will benefit from seeing their work in a different light. NotanIzer is that light.