i dispose old car, why they asked for depreciation, I used milleage as expenses

Mileage has a built in depreciation amount that must be used when you dispose of the vehicle that you must have used on your tax  form. If you took the standard deduction,  for 2020, it was 27 cents per mile driven. For 2019, 26 cents per mile. For 2017 and 2018, 25 cents per mile, 2016 was 19 cents per mile. You can google  IRS mileage depreciation if you need more years.

Here is an example of why you need this:

Paid  $10,000 for car in 2019. Drove it 5,000 miles for work in 2019 and took standard deduction for mileage. 5,000 miles x 0.26=$1,300 of the write off was for depreciation which reduces your basis.

Sell car January 1,2020. Basis in car is $10,000 paid -$1,300 depreciation = $8,700 basis.

• Sell the car for $9,000. You made $300, capital gain.
• Sell the car for $8,000. You lost $700, capital loss.