If I am filing the standard deduction, why do I need the mileage for my vehicle?

I can't be sure in your case, but one reason might be because you are using the vehicle for your own business (Schedule C). In this case, vehicle expenses go on Schedule C, not Schedule A.

Do not confuse the STANDARD MILEAGE RATE  on the Sch C for Vehicle expenses  with the  STANDARD DEDUCTION (vs Itemized deduction) on the form 1040.   Other then using the same word they are not the same thing.