These massive vessels were built to send humanity out into the stars before the advent of super-luminal drive technology.
Arc-Ships were designed to house tens of thousands of people, have facilities to grow food and recycle waste, and otherwise be completely enclosed, self-sustaining ecosystems. They left earth with crews numbering in the thousands, and were intended to travel to the closest stars to Sol that had been determined to have Earth-type planetoids orbiting them. It was projected that by the time they reached their destinations, they would be filled to capacity with residents who had been born aboard the ships. Also known as ‘generation ships’ it was known that these vessels would house many generations of people, and that the majority of them would never see Earth, the Sol System, or even their final destination, during their lifetimes. Although sub-space links did allow them to have regular, real-time interactions with the rest of humanity.
Not one single Arc-Ship ever reached its destination under its own original design paramaters however, as super-luminal drive technology was built, tested, and deployed within decades of the ships launches. Once the technology was deployed, several small, super-luminal vessels were sent out after the Arc-Ships with technology to install in order to make the massive vessels capable of traveling faster than the speed of light.