Quick: what did you have to eat last week Thursday? And I mean everything. Every drink, snack, meal, and mouthful throughout the entire day. Chances are, you can't recall, and that's fine...unless you're trying to analyze problems with your current dietary habits and improve upon them.
Tracking everything you eat is the only reliable way to detect unhealthy patterns and measure your progress in correcting them. (The same is true for fitness, but that's another story.)
But there's more to it than that.