I think its food for the bacteria to get started with, and fresh aquasoil has a lot of crap in it.... When I bought mine lots of protein film came up and out of the water so I guess it helpsnto absorb that, too. After that it's just more surface area for bacteria to grow on, because charcoal/carbon is only good for a limited amount of time. Also absorbs tannins,
That's why I did not buy tourmaline BC. I just added 100 mls of matrix carbon (spherical so it blends in with the aquasoil if it gets mixed up ever) and crushed my own tourmaline into sand and added it. I did not like the fact they they don't tell you what ratio tourmaline bc has of carbon to tourmaline in the 100 gram bottles they sell, so I know exactly what I put in my system.
For a terrarium it serves as a drainage layer I guess, and in the olden days they probably thought it was good to keep the media fresh but carbon only lasts a month at best (I think seachem matrix claims six months?) but having set up terrariums before (more like vivariums minus the plants) you should Make a false bottom so you can water as much as you want and drain it out when you want and nothing becomes foul.
Charcoal is a good soil additive to any terrestrial soil mix really, since it's light and airy and adds some chunkiness. I add horticultural grade charcoal to my orchid and carnivorous plant mixes. It absorbs trace elements for a while then just becomes chunky soil additive. Pumice is good, too. Pea gravel would work if you don't want to use charcoal but a false bottom is the best.
On a side note, I just ordered a half pound of tourmaline on eBay for THREE bucks! If you want to use it, go to eBay and type in schorl! You can get like four pounds of the stuff for what it costs for 250 grams of the Ada stuff, but you won't get the pretty box or green filter pouch it comes in.