Capital Gain Tax Valuation is the process of determining the fair market value (FMV) of a property or asset as on a specific date to calculate capital gains tax under the Income Tax Act, 1961.
It is required when:

Selling a property acquired before 1st April 2001

Calculating indexed cost of acquisition

Filing returns involving capital gains

Claiming exemptions under Sections 54, 54EC, 54F, etc.
As per Income Tax law, for properties purchased before 1st April 2001, you can substitute the actual purchase price with the fair market value as of 1st April 2001 for capital gains calculation.
Only a Government Approved Valuer like Dr. S. N. Bansal is authorized to issue legally valid valuation reports that are accepted by:

Income Tax Department

Chartered Accountants

Tax tribunals

Legal authorities
Residential or commercial properties

Land or agricultural land

Jewelry or gold

Shares or mutual funds (in special cases)

Paintings, artworks, and other capital assets
Yes. A professionally assessed FMV may help reduce your capital gains liability, especially when the property value as of 01.04.2001 is significantly higher than the original purchase cost.
It is not mandatory to attach, but the Income Tax Officer may demand it during assessment. Having a certified report from an approved valuer protects you from future scrutiny.