The point of a well-formulated ketogenic diet is to eat in a way that keeps insulin as low as possible for as much of the day as possible. Eating in such a way allows insulin-resistance, diabetes, hypertension, cardiovascular disease, and many other metabolic conditions to heal, or at least prevents them from getting worse. The reasoning behind this way of eating is that elevated blood sugar and insulin are both detrimental to the human body (in a number of ways) over time, so that eating in a way that keeps those levels low is helpful to the body.
Diet advice from the hundred years before the introduction of the governmental dietary guidelines tended to note simply that (as “every woman knows”) sugar, grains, and starches are fattening and should be avoided, while leafy greens and certain other vegetables don’t appear to be a problem, so you could eat as much as you wanted of them. Fruits, being sugary, and tubers, being starchy, were to be avoided.
This advice may still be relevant for healthy people, but so many people these days suffer from metabolic disease that some attention to exactly how much carbohydrate we are eating may be warranted. The upper limit on carb intake recommended on these forums, 20 g/day, was set by the Two Keto Dudes to provide success to the vast majority of people undertaking this way of eating, with the understanding that people with really bad insulin-resistance may have to lower their carb intake even further.
If you find that your health improves by eating less than 20 g/day of carbohydrate, then that is the desired effect (fat loss is merely one of the very pleasant side-effects of this way of eating). Your body may very well be able to tolerate a higher level of carbohydrate intake than 20 g/day, but we strongly recommend waiting until you are fully keto-adapted before you begin to explore where your carb threshold actually lies. It is possible that, once you are accustomed to eating this way, you may not even desire to increase your carb intake above 20 g/day.