My assumption of why floating rotors are better used to be that it's because it lets the rotors "float" or move around and that would help them somehow get better friction with the brake pads in the caliper.

Then last year I installed some front EBC floating rotors on my K100 and it kind of bothered me that despite being called floating there didn't seem to be any appreciable movement between the carriers and rotors like there is on the front discs of my 4V Ks.

So I did some poking around to find out what the real purpose of floating rotors is. I found that my assumption was wrong and that the actual purpose of "floating" rotors is that as brakes heat up during braking the different metals of the carrier and rotor expand at different rates and the bobbins in "floating" rotors accommodate this difference to keep the rotors and carriers from stressing each other which can cause distortion or warping.

I had thought the same as you, with the addition they might stop judder

I understood the purpose was to keep them from warping, as you described Duck.  I've had EBC floaters on my 1983 R100RS for many years.  I'm gonna guess it's coming up on 8 or 10 years now. There is some pulsing at the lever these days so I'm unsure if the floating aspect of the design is effective or not.  However, since they're quite old, it could be expected that the rotors are not terribly true anymore.  
As an aside, let me declare: rotors are shockingly expensive!

Yes, the OEM rotors are expensive BUT:

Motobins has Brembo replacements for 125 UK pounds each. (Their site lists them as Grimeca but they come as Brembo in a Brembo box.)

James Sherlock sells aftermarket rotors (rotors only, no carriers or bobbins) for only 44 UK pounds each.

The reason for having floating rotors is that calipers on most bikes are fixed with pistons only on one side. If the rotors didn't float the piston would put undue pressure on the rotor.

Cars either have floating calipers or multiple pistons on the offsetting side to handle the pressure.

For example, my 1965 Mustang 2+2 has 4 piston calipers. Two pistons on each side of the rotor.

From EBC's website:

Do not confuse floating rotors with rotors that feature loose rotors blades or buttons. You should not be able to move either the buttons or rotor blades from side to side, if you can, it is highly likely that pad “Knock Back” will occur and you will end up with spongy brakes. Float means outside to inside heat expansion ability. That is what EBC X and XC rotors deliver and no other rotor can.


https://ebcbrakes.com/product/floating-mc-rotors/