The space between things in space is quite large, so you're going in a single direction for a while. The thrust from the spacecraft is also used to exploit gra

The space between things in space is quite large, so you're going in a single direction for a while. The thrust from the spacecraft is also used to exploit gravitational energy (slingshoting around planets) for the most part, from my understanding. In Sci-fi, it's generally understood that very fast crafts (0.1c and up) will need to reverse and apply thrust opposite their direction to slow down for a very large part of the voyage (up to more than half) - so could be a good idea to be able to flip around and apply thrust the other way too.