Apparently floating rotors don't warp the way solids do. A solid rotor will heat the outer portion while the inner portion is fastened to the hub. Yes heat does transfer but the big temp difference between inner and outer combined with the fact that the rotor is rigid restricts expansion which sometimes causes warpage.
A floating rotor separates the outer ring from the inner with rivet type fasteners. The outer ring is in contact with the brake pads and the heat generated is kept from the inner ring in contact with the hub. The outer ring can expand with no restrictions.
A floating rotor can also move to compensate for a slightly crooked hub and as Ian stated maintain pad contact.
