By trading daily and consistently backtesting, you'll start to see your mistakes and identify the principles you're not fully following in your own strategy. Backtesting lets you put your strategies to the test—it’s like a shortcut or an accelerator.
I can see your point, but what if the 'mistake' that I made gives me better results? Does this mean my strategy is a failure or does this show that I'm adapting or betting correctly? I'm not saying this to discourage people from backtesting though, just that it's probably a good idea to not blindly judge every unplanned action that you take as a mistake. Maybe more experienced traders can give insight into this.
That being said, the scenario that I talked about is rare, at least for the average joe. Most of them usually become too greedy and don't stick to their plan instead of improving on the fly because the market becomes aggressive and the patterns change. CMIIW.