Body roll in front crawl is useful for many reasons, but the most important is probably to make life easier to breath – if you move your hips and shoulders round then head movement is reduced to a minimum. It also allows for a longer reach in your stroke (forward and back) and using the larger muscles of the back (as opposed to just using your shoulders).
How to improve your rotation and position in freestyle
I tend to encourage people to try and move from the hips, keeping a straight line from the hips to shoulders and then to the hands at full extension, as this allows your core to do more of the work than relying on the shoulders, which are much smaller.
Depending on your relative comfort in the water, and your strength and stability, you probably should be aiming for between 30 and 60° of rotation to each side. Everyone’s different though, so it’s something you can experiment with to find what works best for you. Most drills (side kick, 6-1-6, etc) work on the basis of exaggerating the movement that you’re aiming for (in getting to 90° to flat), so that when you come to swim half that distance it feels easy.
My normal coaching cue would be to think about rocking your stroke from the hips, and shifting your hips down or up as the corresponding hand enters the water. This might also help smooth out your rhythm as you work on it.