Demonic tongue is rarely spoken, and mostly used in writing. It contains power, inherently "using" magica as it is written and spoken. To speak the words means that the messages contained are richer, more powerful, and can even contain truths beyond what speaking normally could confer. However it does use magica in the process, and is generally seen as wasteful unless the demon wishes to convey great meaning.
Demonic tongue when written is considered extremely useful, as it can contain inflection, tone, and insight. Studying from a Demonic tongue book on Magical Arts conveys huge amounts of the writer's understanding, to the point that humans who have learned and attempted it can sometimes experience hallucinations, distressing dream states, and even suffer seizures.
It is yet another arsenal in the armory of Demonkind, and a powerful tool for those that wish to communicate with them.
-A beginners guide to the Cursed Demonic Tongue, a banned and extremely rare text