Yes it's not mandatory, but this sideswitch rests under your thumb, and you squeeze stronger in order to trigger the 'click'. I find it inaccurate for actual drawing because it makes me slightly move the pen point itself (both on press and on release).
Also, it means that during a brush stroke, I have to hover the tablet without touching it, it feels very unnatural. Painting with BG color should not be a complete different mechanism than painting with FG color.
This is why I find the ALT modifier much more natural, and it fits well with other keyboard modifiers for your left hand :
- hold SHIFT : stylus is constrained to horizontal/vertical axis
- hold SPACE : stylus pans view instead of drawing
- hold ALT (if you enable it) : stylus performs right-click function of the current tool instead of the left-click function.