Help me expand my perspective.
After doing this for a while I started developing stomach problems (unrelated to my change in diet), and I needed to ensure that I was meeting my minimum requirements for fibre too. All of a sudden you end up with quite a tricky multi-dimensional optimisation problem, and it was actually quite a fun intellectual exercise trying to meet all the constraints, obviously without hating what I ate.
There are all sorts of reasons that you might want to minimise or maximise some micro- or macro-nutrient. Any athlete will want some sort of control of what they eat. Perhaps you want more magnesium for better sleep, and you'd like to get it from actual food rather than supplements. Calorie restriction while keeping protein high is a common one for losing weight. Almost no-one eating a western diet gets anywhere near enough fibre, and any one of various health scares might make you aware of that. Tracking what you eat permanently is a serious commitment that is probably unnecessary for most people, but I think it's a good thing to try, at least for a while. I tend to go back to tracking periodically when I want to get some aspect of my diet under control, but I don't do it all the time.
Too much sodium / low potassium are common in most diets. Low magnesium (sleep issues), low zinc, low fiber etc are a few others - a common remedy being nutrition supplements. Better tracking allowed me to eliminate additional supplements from my diet (except Vit D, which is hard to find in natural sources).
Famnom suggests FDA RDI defaults, but users are free to choose what they would like to track.