Even where ladder legs are cut accurately and are planed or sanded smooth, they are likely to damage a wood floor. A leveler with a soft face should instead bear the contact force, and will cope with any cutting errors. A soft face also gives traction. A ladder will sometimes kick out at the upper swinging-section hinges, where one leans forward, standing upon the second section. The traction adds to forces maintaining alignment.
Leveler legs also give small added floor-ceiling range of a ladder. Range adjustment of perhaps an inch can really matter, with unlucky proximity of the bottom step, to the floor. Unlucky landing will more-importantly be controlled by ladder door angle adjustment.