Charcoal takes up root exudates, so if a tree is bleeding heavily ( juices which can hinder new growth), or something in the soil is producing a nasty thing that's detrimental to the health of the plant, charcoal can take it up. Microbes nest in that charcoal and they can break down some of those nasties sometimes.
Sometimes those exudates just need time to decompose into harmless substances, and charcoal can be a temporary vessel where the plant roots can't reach it while it breaks down.
Same as with eating spoiled foods, charcoal can take up the bacterial nasties that you shoved down your throat and keep them locked away from your body until you poop them out. It is indifferent though, and it can and will take up nutrients, medicin, drugs, enzymes, proteins, anticonceptives, and all that as well, rendering them useless.
Is charcoal needed in the soil? No.
Can it be beneficial? Certainly, for the above mentioned reasons. But there's also this: As the charcoal breaks down, it forms new hydrogen bonds and breaks others, potentially becoming a food source for the soil food web. Not all of the charcoal will do that, some will stay inert, but even in an inert state is has a surface area of a couple square meters (because nanotubes!).. A safe and stable place for microbes and fungi to reside in.
In plant tissue culture, charcoal is often used to deplete the media of hormones or to "snag" nasty chemicals, making them unavailable for the plants for some time. Change the medium, and you take away the bad stuff along with the charcoal, and you'll be left with happy and clean plants.
