As NORDS have pointed out, there can be some rate issues for some...
But the depreciation is the matching as Red Hawk said... in business, you buy capital assets that have more than a one year benefit. You get to deduct ordinary and necessary business expenses. So, if you buy paper to print on, you get a deduction. The same is for your capital assets, but since the cost is high and the benefits 'long', you only get to deduct the part of the asset you 'used'. So a computer might be 3 years of value to you, so you get to take 1/3 per year. (I am not up on if you get to do double declining etc.. but that is another question)...
For most assets, the actual value of the asset goes down... think computer, car, truck, things like that... but some assets actually go up in value... buildings, houses, etc.
The thinking is that we do not know if the asset will go up or down in value... think of a sport stadium... how many have you seen blown up lately as they had built a new one down the road... there is no value left... so the tax code assumes ALL assets will be worthless in the end... and if they are not, then you should not have received the benefit of the deduction over the years and now should pay the taxes back on those deductions...
We can only know the answer when you sell the asset....