People always talk about ketone bodies forming in the blood when low carbs are ingested (ketone bodies are not a good replacement for glucose in providing energy for the nervous system and brain). This actually isn't the case. Ketone bodies form after the glucose in the body runs out. Meaning, there are no more amino acids to convert to glucose, thus drawing upon fat for the main source of energy. If the protein intake is high enough, the body can continue converting amine groups to glucose via gluconeogenesis. You want this process to keep up so you'll have glucose for your brain/nervous system/etc.. Ketone bodies are nowhere near as efficient as glucose for supplying energy to the brain. Glucose is essentially "brain food". So if you are going to skimp on carbs, keeping the fat and especially protein higher, you can keep this process from happening. You also want the protein intake higher so the protein won't be sacrified to the body over your muscles.
And this is a good example of the difference between ideal and essential.