Notan comes from the Japanese and it roughy means the play between light & dark. I use a 2 value notan to help me simplify & see if there is a strong & pleasing design. I rarely paint in 2 values but it's a very quick exercise. If the composition works in black & white, then I'm good to go. To take less than 5 mins. to do a simple exercise to avoid a bad composition, seems to me a no-brainer. I believe it's an invaluable tool & should be part of your painting process.