Students attending any of my workshops will be familiar with my thoughts on tonal relationships, and how important I consider them to be in painting. If I had a list of priorities, in making pictures, I would put tonal values just beneath design and just above colour.
I often start my workshops with a few 'loosening up' exercises, and monotone studies are a good way to begin.
This small study of a glass bottle was a class demonstration. I wanted students to concentrate on just 3 tones: light/medium/dark, plus white for highlights. This exercise really helps them to stay focused, especially if they don't stray from the idea of just 3 tones.
Of course, in watercolour, your lightest tone is the white surface of the paper. Being a transparent medium, every wash that is applied makes a darker tone. Less water in the washes, means darker tones.