Hi Diwa,
I was not able to understand fully what you are asking. Are you asking why we assign depreciation key to income tax depreciation area?
Financial reporting is done differently for shareholders and tax purpose. In case of shareholders the motive is usually to show good profits so as to please them (less profit in case company wants to give less dividends etc. Read any good book on financial reporting). One way to do this is by adopting such accounting policies where depreciation is shown less thus Total Asset balance in balance sheet is more resulting into higher retained earnings.
However, when it comes to tax then one will try to pay less tax. For this one will show higher depreciation thus resulting into lesser profit and lesser taxable income.
SAP also provides the facility to generate financial statement for reporting and tax purpose and in case one wants to differ in the way depreciation is calculated in both the reporting then they can go for different depreciation key in both of them thus there is provision of assigning depreciation key to income tax depreciation area.
I am not sure if this answers your question but let me know if it is helpful. Rephrasing your question might help me to understand it better.
Regards
Pankaj Pareek.