Moving through space in The Lost Fleet universe, that is mostly empty, and where non-empty spots would be suns, planets, moons etc., involves great distances and days of time, when inside a solar system or in jump space between them.
In our case we're talking about the Fleet consisting of hundreds of ships, which are military designs, meaning highly maneuverable and fast units, heavily armed and armored in some cases, which means they will have a lot of mass. Propelling that mass requires a lot of energy. Energy generation requires fuel.
In order to propel such a starship to, say, 0.2c requires significant energy and the higher the acceleration the more fuel it requires.
But in space there is nothing to slow that vessel down. So, what it has to do to change direction? Or better yet: turn around left and down?
Exactly: basically exactly the same thing it did to attain the speed and direction along the plotted course in the first place, and twice that if it is "3-D" turn-around.
But if we want that ship to not be constantly subject to stresses of high-g maneuvering (and it is said in the books that any ship can basically loose propulsion under maximum load), we have to do low-g maneuvering.
Which literally means that instead of turns of radius of hundreds of kilometers, there will be turns with radius of hundreds of thousands of kilometers... Which will look on the plots like lines "curved in a graceful arc"...
So, bottom line is still the answer I gave initially: fuel conservation. Also to avoid unnecessary engineering casualties due to stress of constantly travelling at maximum power. So to also conserve spare parts, to avoid delays and more fuel consumption to slow down in order to keep formation. And not to leave anyone behind.