If you don't have any trading experience at all virtual trading is where you can learn and practice without the emotional stress of losing money. You can try various strategies and you can get familiar with the market. You can also learn how much "money" are you able to make. Virtual trading can help you to fine tune your trading style or establish one.
If you have some experience this is the place where you can test various trading strategies simultaneosly. Trading trends is relatively easy but if you want to learn how to trade market tops and bottoms or trading ranges virtual trading is where you should start.
Even in your virtual trading account you shouldn't gamble, treat it as if it was real money. Remeber what you learn there you want to apply in real life whit real money. If you take virtual trading seriously you will able to learn a lot about your risk tolerance and comfort zone. Once you have a solid foundation you can move on to trade with real money.