For a business there is a difference between cash flow and tax profit/loss calculations. If you buy a $50,000 car, it is listed on your balance sheet as an asset and it goes down in value over time on the balance sheet. During that time, you are hopefully getting more value from the car than the amount of asset you are losing. From a cash flow point of view you spent the money when you bought it. The money needed to be in your checking account. But, your company is not allowed to "expense" (write-off) the car as a business expense all in that first year because it still has value on your asset sheet.
The idea is to show the profitability of your company more accurately. You end up getting to write-off the entire $50,000 from your profit, but you do it a little bit each year.
One of the provisions of the Bush 2001 and 2002 tax bills was the ability to accelerate depreceiation on business purchases (to allow you to capture the business costs sooner and encourage people to buy things for their business and stimulate the economy. And it worked.
Hope this helps. I just tell my accountant each year what I spent on capital expenses and when they were put into service, and he sticks them in the software and it spits out the proper depreciation on the corporate tax returns.
The opposite is actually true. You currently lease because the depreciation rules on autos limits the amount you can depreciate a year. Under a lease, you can expense every penny you spend on it. The rule is dumb. It was intended to stop people from buying $50,000 BMWs and depreciate them as a business expense. But now what is considered luxery is a Ford Escort. Then again, there were great rules last year that allowed you do immediately depreciate giant SUVs.
Actually I don't see that your question was answered, but under the FairTax, there is no such concept as "depreciation" required for tax purposes (since income is not taxes) and you are free to make the business decision of expensing an acquisition as meets the needs of your business the best - in your view.
That's one of the marvelous aspects of the FairTax ... it's called FREEDOM.