"Formless" probably means there's not a lot of stiffness in the sidewall. That lack of stiffness will mean (a) it deforms better when you hit gravel, giving you a smoother ride, and (b) you won't lose as much energy to bending the sidewall which you don't recover (like a spring), meaning you'll roll further and faster with the same input energy from pedaling than you would with a stiff sidewall.