This video is exactly what Isaac Arthur says it is in the beginning. The video explores the common trope in sci-fi of nomadic spaceships that left their homeworld. The Battlestar Galactica franchise is the fictional example I am most familiar with.
There can be many reasons why a bunch of individuals would band together and spend long stretches of time traveling in space. As the video points out the reason near the end of the trip might not be the same as the original reason in the beginning. In these scenarios most of the time the colony spacecraft are generation ships. Without faster than light travel it could take hundreds or thousands of years for a spaceship to reach another star. During that time the society aboard the ships could change and evolve. The population of the colony can also be many times greater when the destination is reached than when the migration began.