Silmaril is the human central computer as compared to Yggdrasil and Nidhog.
Originally, the Whispered Network was a bunch of laptops connected together periodically, sometimes coming offline, sometimes coming online and connecting to an unorganized junk pile of information.
With the Whisper of Souls Tessa's got working, they were able to upgrade to a series of laptops connected together constantly rather than popping on and off, but are still limited to the power of those laptops so what each individual is working on is limited to their own personal processing power.
Once they add the Silmaril without the Whisper of Souls marks, then it's like the laptops are giving a command to a supercomputer to sort through the pile of junk and waiting for it to bring back an answer, perhaps a bit faster or able to perceive more of the chaos, but still maddening.
With Silmaril and Whisper of Souls marks together, it's like a bank of constantly active laptops with an open door to a supercomputer.
Without Silmaril, everybody's running on their own private battery.
With Silmaril, then everybody is suddenly connected to a palladium reactor.
Silmaril generates massive amounts of life energy, though it hasn't had time to grow much yet, still it is a lot of energy. It should already have more than enough energy for the small population of Whispered that are likely to be around right now. This is what's going to allow humans to reach Category 1 levels.
It provides processing power and a place to store chi patterns and knowledge so others can access it easier. And also allows the connected minds to interact much more quickly and easily.
Facial Marks Edit
Primarily, they provide a firewall that makes the marked person a lot more difficult to take over by possession or hypnotism and the like. This also means that the telepathic-like aspects of Resonance are blocked and the Whispered will lose their semi-telepathy (save special cases like Naiki/Tessa and Kaname/Eija). Like the Gods and Demons, the Whispered are likely to be mostly non-telepathic save special instances: such as borrowing a telepathic ability from someone else in the area. Since a true telepathic connection is still possible, though getting past conversation will be difficult.
It enables the Whispered/God/Demon to maintain a constant presence on the network. Essentially, a part of Tessa's mind is always in the Whispers now. Originally, this would eventually drive a Whispered mad to spend too much time, since it requires too much effort to filter out the barrage of information unrelated to why you are there. However, the marks act as a sort of filter that keeps her presence to a mostly unconscious or subconscious level and only lets in the information she wants when she wants it.
Divine facial markings are blue, demonic facial markings are red, and Ainur facial markings are green.