Amounts paid as advance tax and withheld in the form of TDS or collected in the form of TCS will take the character of your tax due only on completion of self-assessment of your income. This self-assessment is intimated to the Department by way of filing of the return of income. Only then the Government assumes rights over the taxes paid by you. Filing of return is critical for this process and, hence, has been made mandatory. Failure will attract levy of penalty.