It depends. 24/7 support is essential to some investors, while others may be completely fine using online chat during regular market hours or receiving an email back within one business day. That said, most investors neglect to think about a market crisis like a flash crash. In our experience, it certainly doesn't hurt to have reliable customer service available for whenever the need may arise.
A broker is simply a licensed person through whom you can buy and sell these stocks. When you use an online platform, it essentially acts as an online broker. If someone wants to buy any product, he can contact a brokerage service. They would place an order and let him know the market price of the product. If the investor is interested, they would include a commission for themselves and process the order forward. If the user wishes to buy stock, the broker would forward his order to a stock exchange, which would approximately take 3 days to complete including the money exchange between the broker and the investor.