Including both investment grade bonds with higher-yielding ones in an investment portfolio can help reduce overall risk exposure because their returns are usually negatively correlated. When riskier, growth-oriented credit assets such as high-yield bonds fall in value, government bonds and other interest-rate-sensitive assets usually rise, and vice versa.
Because negatively correlated assets tend to take turns outperforming each other, investors can sell the outperformers on one side (for instance, high-quality high yield) and buy the cheaper bonds on the other (for example, Treasuries). That approach has historically tended to increase returns over time.