my 2 cents answer...
the alt crz mode allows 50' change in altitude before actually moving any a/thrust setting
this is to relieve stress on the airframe and its contents...
you can disconnect it as well, setting the recommended turb n1...
meaning like 'don't touch anything in turbulence unless really needed'...
at higher altitudes fly mach i/o ias as a result of air compressibility...
meaning less a/thrust response for deviation unit,... thus more comfy ride.
happy landings all