Just for fun: This is my mathematical estimation why we do sketches.

Taking 30" X 22" watercolor as an example.

Assume average time to complete a complex 30" X 22" watercolor
8 hours of painting time

8hours = 8 X 60 = 480min

One year, let's say, we paint 12 'GOOD' paintings = 12 X 480min = 5760 min
However, the success rate of making a reasonablely good 30" X 22" watercolor
is as low as 50% or lower.

Taking 50% as e.g => Needs to paint 24 paintings to get 12 Good ones

=> Total time taken = 5760 X 2 = 11520 min
One sketch on average takes 3 mins

=> 11520 / 3 = 3840 pages of sketches can be made in a year
if I don't waste time to paint.

If for 3 years I just sketch and don't paint => 3840 X 3 = 11520 pages of sketches.

In my life, I have not met any artist that has done more than 10,000 pages
of sketches and still has not acquired high level of skills.

=> It only takes as short of 3 years to be an excellent artists. Isn't this cool!

That's why I always focus on sketches.
Share this with you painter friends and they will stop painting for awhile.

Chinese Painting master Li Ku Chan (李苦禅) says" Sketching trains 'quick hand', 'fast eyes' and 'hand and heart' can hence work together, arriving at brushworks that are: FIRM, ACCURATE & FIERCE!"

'Sketch Theory' by Ng Woon Lam