You have found the house you want and everything is sorted out. The price, all conditions and date of transfer. The contract is signed. Usually one of the conditions met is the condition that you are granted a loan or mortgage. In general the bank will insist on an independent valuation. They may and will not rely on the judgement of a selling, nor buying, estate agent as this always has the inherent risk of not being completely impartial. So this valuation is done by an independent appraiser.
This is an extra built in safety measure for you. If the value is lower that estimated earlier then you can annul the contract based on the explicit condition of being granted the loan. The bank won’t agree with the mortgage. Even if you don’t need a loan it could be a good idea to enter the condition in the contract that the value is assessed by an independent appraiser.