Magic Circuits are an essential “organ” for the use of magecraft, spread through the body in the form of a fixed number of pseudo-nerves. One reason behind the mages’ jus sanguinis doctrine is that they strive to increase this number, even if by just 1.
Likewise, the Magic Crest is a symbol of the accumulation of experience in a mage’s bloodline.
However, normal Magic Crest are not provided with biological functions like the one you’re currently seeing. Instead, they are “artificial hearts” designed by each family to be passed from parent to child.

The Crest’s creation is supposed to span centuries or millennia with each holder adding their own contribution to it. It suddenly proliferating outside the holder’s body is normally unthinkable.
But the boy immediately figured out what was this thing constructing himself around him and what was it for.
The magical energy in the city’s rich air, or perhaps the land’s dragonvein resources allotted to the Holy Grail, were assembling themselves together around the boy in a frantic speed.
Borrowing Holy Grail War terminology, the sheer of magical energy concentrated in one place could plainly be described as a “Noble Phantasm”.


Fate/Strange Fake: Volume 7, Chapter 21: "Beings Shaped in Man's Image"