Because it is profitable! This alone is a GREAT reason to use Grid Trading. It is a profitable trading system, it’s been around for quite some time, it has proven that it can make money.
Because it is automated! Nowadays there are excellent tools to automate your Grid Trading system. We offer one ourselves – this is GRIDAlgo. This EA is a part of our Grid Trading Course, we explain each and every feature it has during the live webinars and we show tons of examples as well.
Because it is versatile! You can trade it on any currency pair, on any timeframe, on any account size. In other words you can adjust a Grid Trading System to your trading style!
Oh, and did I mention because it is profitable? 
Feel free to check out this other article of mine (below), it gives a bit more details about why you should use Grid Trading: