If you want to make money in the price movements of instruments, rather than purchasing physical assets on a higher price and with a minimum share requirement.
To take advantage of swift fluctuations in the underlying instrument or security. This is popular with 30 seconds investors looking to profit from day break and overnight movements in the market.
To take advantage of leverage and spread capital across a range of different instruments rather than tie it up in a single investment (note: this approach can increase risk)
As a risk management tool to hedge exposure