I'm quite good at figuring out the market. Why should I start using a trading system to trade?

It's possible that you have a strong mental attitude and wont be affected by the market. But trading the markets is a emotional rollercoaster and at one point or another you will be affected by it. Using your insight when trading can work and maybe you'll never need anything else, but that will be more a matter of luck than anything else. By following a trading system a trader blocks out the emotional factor and is not affected by the markets emotions. But one thing has to be kept in mind. Any trader has to feel comfortable with the system he's trading otherwise he will never succede. Not all systems fit all traders. To find out what trading system fits indivdual trader, nobody can tell but the trader himself.