Just collected this information and put it into bullet points to remember easier:
When not training, we require only minimal amounts of glucose, most or all of which can be supplied by the liver as needed.
Excess glucose from carbohydrate intake (especially from processed grains and sugars) is the primary culprit in obesity and in many disease processes.
If you constantly supply your body with glucose then your genes get used to using glucose as a primary source of energy, storing what extra it gets as fat in times of plenty. If you eat more fat then your genes reset to using fat as an energy source like we evolved to do, and fat stores get used for everyday energy.
There is actually no requirement for any “essential dietary carbohydrates” in human nutrition. It’s possible to live a very long and healthy life never consuming much – if any – in the way of carbs, provided you get adequate dietary protein and fat. The same can’t be said for going too long without protein or fat. Cut too far back on either of those macronutrients and you will eventually get sick and die.