prefix 66

I still don't understand what situation we need to add 0x66 ?

adc al,0x12

bit 16 = 14 12 
bit 32 = 14 12 
bit 64 = 14 12 

adc ax,0x1234

bit 16 = 15 34 12 
bit 32 = 66 15 34 12 
bit 64 = 66 15 34 12 

adc eax,0x12345678

bit 16 = 66 15 78 56 34 12 
bit 32 = 15 78 56 34 12 
bit 64 = 15 78 56 34 12