My understanding that either a relic or a cathedral was the middle ages version of a moon shot. It placed you "on the map" and just having one turned your city into an economic engine and a seat of power ... well, more than it was, anyways, since it took serious resources to build one.
Literally, a cathedral is the seat of a bishop. The Paris cathedral was important because it was the home of the French kings. Saintly relics (or in Notre Dame's case of Jesus) served several purposes. They served as pilgrimage attractions, which like all tourism, brought in revenue. Relics were thought to be a "power up" for sacraments, and lent an extra air of authority to the bishop's pronouncements. Cathedrals also served as public spaces. They were the only buildings large enough for the whole community to gather, and frequently hosted market days and merchants. They were also educational. Most people of the time were illiterate. Even Stephen and Mathilda, 12th century English royals, couldn't sign their names. A cathedral was in effect a picture book, via statues and stained glass windows, telling religious stories. Lastly, they were status symbols for nobles and guilds who endowed chapels and were interred there. Even today, notable people get buried in Westminster. The modern equivalents are naming sports stadiums and university and office buildings.
Medieval masons were the "general contractors" for building cathedrals and other stoneworks, like castles. This is similar to how carpenters are often the general contractors for house building today. Stone and wood respectively are the primary construction materials. Masons were also the architects, because architecture wasn't a separate field yet. These projects took so long that the masons built on-site housing, called "lodges" where they lived, and also oversaw the work. Typically the bishop, or one of his underlings was responsible for raising the money for it. The unskilled work was often supplied by the community, with different wards or parishes taking turns supplying laborers. Didn't take much skill to walk the great wheel used for lifting stones. Basically a giant hamster wheel: