Why is protein considered the important focus for weight loss and maintenance in the short and long-term?  In a word, satiety.

Animal based protein is nutrient-dense; when in included at adequate levels in your diet, it is much easier to meet nutrient needs each day.

Most calorie restricted diets not only reduce calories, they significantly reduce protein intake. This causes the hunger that is often reported once an individual is into their lower calorie diet after a few weeks. While starvation does work in the short term, long-term 95% of people who lose weight will gain it back within three years, 99% will within five. Those are some staggering statistics.

When you do not eat enough protein, you are starving, slowly but surely; but here's the crazy thing -  that also holds true even in a state of excess calorie consumption.
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Protein is a really neat macronutrient, not only does it provide us with essential amino acids, it also is used throughout the body to repair and build muscle, cells and tissue. Amino acids are critical to maintain our health, but they're not really a great energy source - when the body's energy sources are low, it begins to degrade proteins for use as an alternative energy source. Amino acids can be classified as glucogenic or ketogenic.

Glucogenic amino acids can be degraded to pyruvate or an intermediate in the Krebs Cycle. They are named glucogenic because they can produce glucose under conditions of low glucose. This process is also known as gluconeogenesis, or the production of "new glucose." Amino acids form glucose through degradation to pyruvate or an intermediate in the Krebs Cycle. The intermediates can then be converted to oxaloacetate, the main precursor for gluconeogenesis. (glucogenic amino acids: alanine, cysteine, glycine, serine, threonine, tryptophan, asparagine, aspartate, phenylalanine, tyrosine, isoleucine, methionine, threonine, valine, arginine, glutamate, glutamine, histidine, and proline).

In contrast, ketogenic amino acids can produce ketones when energy sources are low. Some of these amino acids are degraded directly to ketone bodies such as acetoacetate (leucine, lysine, phenylalanine, tryptophan, and tyrosine). The other ketogenic amino acids can be converted to acetyl CoA. Acetyl CoA has several different fates, one of which is the conversion to acetoacetate. Although not a preferential energy source, acetoacetate can be metabolized by the brain and muscle for energy when blood glucose is low and acetoacetate cannot be used in gluconeogenesis, since acetyl CoA cannot be converted directly to oxaloacetate.

The above is what happens when you consume adequate protein each day.

So, what happens if you consume too many calories and/or too much protein?

Basically, when energy sources are high, both glucogenic and ketogenic amino acids are converted to fatty acids through the intermediate acetyl CoA. Other amino acids that are degraded to intermediates in the Krebs Cycle are siphoned off into the production of urea, a nitrogenous carboxyl compound that is filtered through the kidneys and secreted in the urine.

Put another way, you now have fatty acids that can store as body fat.

It is for this reason one should not consider a controlled-carb diet as an all-you-can-eat buffet, just hold the carbs. Whether you're new to carbohydrate restriction, or a long-term veteran, you need to know how much protein you need at minimum, and also understand where the maximum is for weight loss and weight maintenance.

How does one know how much is enough and how much is too much?

It depends largely on weight. In fact, it is really all about how much one weighs. Protein requirements are based on body weight because body weight takes into account mass, lean body tissue and structural maintenance.

The more you weigh, the more protein you need; the less you weigh, the less you need.

The absolute minimum, to meet EAA requirements is considered 0.8g/kg body weight. On a carb-controlled diet, one does require more to fuel gluconeogenesis and most agree that protein requirement ranges, from 1.0g/kg to 1.5g/kg.

A good middle maximum is 1.2g/kg if someone is active. 

The chart below provides protein requirement and upper protein range for those restricting carbohydrate in their diet.