Again, fire is not the source of the heat. Anyone who has ever used a Bunsen burner knows that you want as little flame as possible to achieve the highest possible heat. Coals are hot. They are incredibly hot, and they generate the heat which allows fuel to be consumed and generate fire.
People have misconceptions about fire and coals and tend to think that coals are the result of a ‘cold fire’ but this is not the case. Fire is the result of gasses. Not heat. You can reach enormous temperatures without generating flame - basically until the point where air itself will combust, which is an extremely high temperature.
Obviously as there is no fuel coals will gradually cool down, but glowing coals without flame are not a sign of decreased heat, they are a sign of a lack of fuel to ignite. However they remain hot for quite some time and will rapidly generate fire as soon as fuel is reintroduced to the system. Which creates more coals, just as hot, and just as hungry for more fuel. Flameless glowing coals will cook things just as quickly as a fire ignited on the same coals. They are, in effect, the source of that heat. That is just how fire works.
Anyway, charcoal has plenty of uses. You can use it for rudimentary water and air filters. You can use it to create lye for the production of soaps. You can consume it to treat upset stomachs. And of course you can burn it very efficiently. Already, in that short list it has more uses than some of the crafting materials in the game. 
Re: campfires, it is not so much the cooking that interests me. Charcoal is very efficient at retaining high levels of heat without generating flame (see above). Charcoal fires can be used to create heat levels required for things like the smelting of metals. Wood fires cannot reach these temperatures. So you could, in theory, create a system where metal crafting recipes in the game require charcoal fueled fires. Also, campfires made with charcoal would last longer and their ‘Hot Coal’ post-fire status would last quite a bit longer, making for better, longer lasting heat sources, with less smoke, and easier to reignite post-fuel consumption.
I dunno, I like it.
