: Nice explanation by John.

: For the life of me though, I can’t understand why focus stacking
: is being utilised for a relatively flat object like a watch.
: With decent lighting, a small f-stop can be used with plenty of
: depth of field.

: Focus stacking is usually reserved for close-up nature photography
: where the objects have a lot more depth and lighting is more
: variable.

I sort of agree with you. Focus stacking is very useful in macro/micro photography because depth of field is so narrow at close focus. For a long time watch photos were taken with cameras that had tilts and shifts where tilting the lens changed the plane of focus and shifts corrected the perspective. This is not so useful when shooting straight on but perhaps the auction house is just using focus stacking for all of their shots including the straight on shots we see...just because... I don't know and there are other possible explanations. I still have a Nikkor Tilt/Shift lens that I bought specifically for watch pictures. I don't use it much these days.