This is the amount the investor actually receives in his account from an investment potfolio. It is what the investor has to spend. Dividends, interest and realized capital gains are examples of DY. Unrealized gains, computed interest, deferred income are examples of what are not DY.
A company may be the best at what it does or creates. It may have an extraordinary management team, real net profits and an increase in stock price and company intrinsic value. If it doesn't give this back to the shareholder as a dividend or as interest on a bond, then it is a P&L item only. The investor can only speculate that this is true, will continue and that the stock price will reflect all available information about the company.
Income in the form of a dividend check has more value than growth in the form of a change in stock price.