Because you had a depreciable asset that you used for your business.
If you buy a large-ticket item, you're not allowed to take the cost off as an expense in one year. You're supposed to spread the cost over a number of years (unless you take a Section 179 deduction, which it sounds like you didn't).
You should look at last year's depreciation schedule, and set up your asset EXACTLY as it's been depreciated. Turbo Tax will ask you all the right questions to get the depreciation correct.
