As in, "links are the sine qua non of everything."
"It is that city's sine qua non" is often used to convey the specialness of that place; it's uniqueness.
It is more accurately the essence of a place or thing, which is not the same thing at all. The former is the defining characteristic (a landmark perhaps), while the latter is the soul, the heart of a thing.
What is characteristic of a place or thing (what it is associated with) is not necessarily that which makes it wonderful.
La petit prince