The caliper only needs to float if it does not have opposing pistons. If all the pistons are on the same side the caliper has to float in order to keep the caliper centered so you use both pads. Lots of rear brakes only have one or two pistons on the same side and need to have a floating caliper. If they did not have floating calipers as the pads wears the caliper would no longer be centered on the rotor.
Calipers with opposing pistons (2,4 or 6 piston calipers all radial mounted) calipers. Do not need to float because the pistons move on both sides of the rotor and keep the pads in contact by moving the pistons.
Rotors float for a different reason and do not move enough float(or play) to keep a non opposing caliper centered. They float so that the brakes still work well even when they slightly loose shape side to side caused by heat during hard braking. Another reason they float is to increase pad contact through better alignment. They are also less affected by rim deflection during cornering.
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