It’s good advice for several reasons. It’s important to be able to easily step back from the easel, to take in what you’ve been doing from a distance. Sitting makes this long view less likely. If you have to sit, a rolling armless stool is a good idea.
Sitting encourages a kind of torpor that leads to things like not restocking colors on the palette and unconsciously doing little workarounds that may not benefit the work.
It’s hard to get the whole body into the work while sitting. If this is you, it’s best to stick to small canvases that can be seen at a glance, perhaps 8x10 or less. Larger canvases suffer from too close a working distance. It’s why oil brushes have long handles. The brush plus an outstretched arm gives a freedom of movement to the stroke plus an intermediate viewing distance.
Sitting and working closely also encourages a tighter finish to a painting, especially if the reference material is also close. The magic of seeing the painting resolved from a distance, then enjoying a closer view of all the individual brushstrokes—the artists characteristic handwriting—can be lost.
I understand that some may need to sit due to physical limitations. For them I advise setting a timer to step away at intervals. An armless rolling stool to avoid constraining the arm movement and brushstrokes is helpful and also encourages good posture, important to avoiding back troubles. Here’s an article that helped me with this: https://www.npr.org/sections/goatsandsoda/2018/08/13/636025077/to-fix-that-pain-in-your-back-you-might-have-to-change-the-way-you-
A drawing table setup instead of a regular easel may help too, to avoid an uncomfortable arm extension for those with shoulder issues.
Finally, I do sit to paint, particularly at the end of a painting, to paint foreground details. It helps me slow down and control those last few brushstrokes better. I set my timer, sit up straight, make use of that long brush handle, and kick my stool back periodically to take it all in. But stand whenever you can. Be vigorous!
Here’s a link to the Robert Genn letter: Here’s a link to the Robert Genn letter: http://painterskeys.com/howard-pyle-2/