You can open your Equity Trading + Demat Account (cash, derivatives) and Commodity Account. You cannot open HUF, Partnership Firm, and Corporate account through an online process.
Note- Client can buy Equity Delivery but selling will be unblocked once POA hard copy is received.
Power of Attorney is required so that when you sell shares the broker can debit shares from your Demat account and submit those to the exchange. It is permission from the client that enables the broker to process the selling of shares.