Programmatic digital advertising provider The Trade Desk (TTD -1.46%) is the latest company to join the S&P 500 index, doing so on July 18. The addition explains why shares of the company shot up an impressive 6% on the day following the announcement that it would join.The spike isn't surprising; The Trade Desk's entry into the S&P 500 is a testament to the company's solid profitability and liquidity in the past four quarters. Moreover, the addition to the index has led to an increase in demand for a stock ...