Why pay tax when TDS is already deducted from Income?

TDS or Tax Deducted Source simply means Tax is collected on Income when this Income is paid or credited to you. The person making the payment is required to make these deductions and deposit TDS with the government.
TDS on salary is deducted by your employer basis the slab rate applicable to you.
For other types of Income or payments made – like interest payments by banks on savings account, interest payment by banks on fixed deposits, insurance commission paid to you, rent payments received by you etc– TDS rates are between 10% or 20%. These deductions are made by the person who pays you this money and has no knowledge of your tax bracket which is dependent on your Total Income. However Tax on your Income has to be paid at the rates applicable to you based on your Total Income from all sources. TDS is deducted without considering your final tax liability.
Therefore, for finding out one’s accurate tax liability – income from all sources – like Salary, Income from House Property, Income from other sources (interest payments from banks, all residual income not under any other head), Capital Gains, and Business & Profession is totalled and slab rates are applied. From your final tax liability – reduce TDS which has already been deducted on your Income – balance shall be your net payable (or receivable) to the Income Tax Department.
Therefore, when your total income is added up – tax may still be payable by you, though TDS has been already deducted.