Investments can growth wealth two ways: by returning income to you and by growing in value. Expected return is the combination of income and growth you expect on an investment in a single year. Of course investments can also fluctuate in value, and some fluctuate more than others. Investment professionals call that fluctuation volatility. The wider the fluctuations, the higher the volatility.
When you go to the supermarket, do you only buy bananas and beans? If you want a balanced diet, you generally buy a variety of things. That way you get myriad benefits from different-tasting foods -- and all of the important nutrients they contain.