.The PoA gives limited rights to AsthaTrade to withdraw shares from your demat account to settle the sell transaction in your linked trading account. Without PoA, the broker cannot withdraw the shares from your demat account and thus cannot complete the sale transaction.
Signing PoA is preferred but optional. If you do not wish to sign PoA, you will have to manually transfer the shares to the broker for settlement every time you sell the shares. This is done by submitting the paper delivery instruction slips (similar to cheque in a bank account).