Creating a diversified portfolio can help you insulate yourself against risk and grow your wealth quicker. With more investments, the idea is that you will lower the risk of losing all your money. When the stock market is stronger, having stocks is a good thing and when the bond market is stronger, having bonds generate higher profits.
It can be easy to get into a rut with your investments since you tend to stick with what works. Having only stocks and bonds is not diversification. However, it is important that you research other options and talk to your financial planner about some other possibilities.