Acts as an emergency fund - it's good to have at least three months' living expenses should you need it to cover you for any emergencies.
Allows you to take advantage of opportunities - it's good to keep some cash to move money into investments quickly to benefit from market upturns or seize new opportunities.
Stops you from being forced to sell - it's good to keep some spare cash so you're not forced to sell your investments and lock in any losses if markets fall.
Plays a role in a diversified portfolio - asset classes (such as cash, bonds and equities) act differently in different economic conditions. Investors can try to reduce some of the effects of volatility to their portfolio by investing across different sectors and market areas. This doesn't necessarily mean higher growth potential but it can help to deliver a steadier return.