#Warning: Backtesting doesn't often work for indicators, they will trick you, forwardtesting is better in this regard. But for strategies like Price action, Trendline diagram, Fibonacci retracement and breakout and Elliot wave theory, it will work, but they have to be done manually to be accurate.
Perhaps you should've added that it doesn't work with "lagging" or indicators that repaint and not that backtesting doesn't actually work at all. How does one guage the present and future if one doesn't review the past? It's not possible. This is where backtesting comes in. For me, I started with those MAs, Fibs, MACD, until I discovered they lag. Now I rely more on leading indicators of S&R, Trendlines and Pivots. In conclusion, anyone who wants to take trading seriously should backrest their strategies before hand.