It is a nearly universal maxim of science fiction that faster than light (FTL) travel must exist. Let us take a look at why this is universally necessary for the sake of a good story by comparing the size and scope of both our real universe and a few fictional universes to how long traversal would take. However, rather than just using years, let us instead consider how many generations it would take for people to travel that distance. Given that one generation is ~30 years, a few lengths of time exist for scale. Human civilization has been around for 6,000 years, or about 200 generations. Jesus Christ was born around 70 generations ago. Modern humans have existed for around 7,000 generations.
Meanwhile, if we assume that we eventually invent (very very near to) light speed travel, it would take about 3,000 generations to traverse the Milky Way. Even though the effects of time dilation would mean that the experienced time would be considerably more negligible, it would be best if you didn’t have an urgent engagement on the other side of the galaxy–it would have gone through effectively half of humanity by the time you reached it. Going to another galaxy is even worse–the nearest galaxy to Earth (Andromeda) is 83,000 generations away. By the time a spaceship has traveled that distance, the galaxy far, far, away will bear little resemblance to how it was long, long ago.
Fictional universes face the same issue. Star Trek takes place in our own galaxy, so faces the same scale issues. Battlestar Galactica takes place in an area of space around 13,000 light years across, or about 400 generations. Star Wars, while localized to one galaxy, is purported to be 120,000 light years across (~4,000 generations), slightly larger than the size of our own galaxy. Dune reaches into several galaxies with no clear size limit, but conservative estimates mean it will be in the tens of thousands of generations. Simply put, for the sake of a coherent story in science fiction, both heroes and villains must be able to travel faster than light. If they were constrained to only being as fast as the fastest thing, a plot would simply be impossible.