Why use pre and post-market prices to calculate the market capitalization and profits and losses of stocks during respective period?

Extended trading is also a regular session of the exchange, and pre and post-market prices generally respond in advance to financial reports and market fluctuations, based on risk control reasons and with reference to the treatment of the industry, the latest pre and post-market prices are used to calculate profits and losses and market value. When business date switchs, that is, after 00:00 US Eastern time, the closing price will be used to calculate the market value and profit or loss of the stock.