Reduced risk while maintaining rate of return.
Portfolio risk declines sharply with diversification. 4 or 5 assets will half the risk of a portfolio. Gains to further diversification diminish quite rapidly.
Managed fund/market tracker achieves diversification.
Reduction in risk through diversification need not necessarily sacrifice returns. Not all invstments rise and fall together eg bond prices can rise when shares fall, interest rates on cash deposits can go up when shares fall.