CA 540 line 12, but this number is not used anywhere else, why?

I put my number in there, sure, no problem.

But it gets me wondering why? I don't see this number used anywhere in my return. Why ask us to put a number in but there is no refer to it anymore?

CA tax are calculated using Line 13-19

I dont get it, if anyone know anything, thanks!

 

Taxable Income

Refer to your completed federal income tax return to complete this section.

Line 12 – State Wages

Enter the total amount of your state wages from all states from each of your federal Form(s) W-2, Wage and Tax Statement. This amount appears on federal Form W-2, box 16.