You can see here a nib that flexes. A hard thumbnail is a convenient firm surface to press a fountain pen nib (point) against to get a feel of how flexxy it is. (inky thumbnails often identify a person who likes flex nibs).
Why pay more? for a pen with a flexible nib?
A pen with a flexible nib CUSHIONS your feel of the nib on paper. And you can SHADE a line with a flexible nib. SHADING refers to widening a line’s thickness by applying pressure to the nib.
On the following page COMPARE my signature with and without shading as the signature alternates “looks."