1)Lock in fx rates (e.g. if you have to make a future overseas payment and the GBP is currently strong)
2)As tax free, low fees, highly liquid alternative to share ISAs, funds
etc.
You have to be very careful with the operational detail but can be very effective way of investing/hedging.
basic think to watch out for is the stop losss limits and the "gearing element". i.e. it allows you to gain 20k exposure with only, say 5k of cash. SO you do need to keep track of this. Best to get some help from someone who knows about this.
Just thought I should mention as so many investments rip you off re: fees, liquidity, transparency etc.