Arcs are used to give more realism to movements, in real life for istance if you want to pick up something or walk, the movement is like a rotation it follows a certain arc (not all the time, it depends on the movement). In animation in order to make the same movement look realistic, drawing an arc could help, the movement of the animated object should follow the arc.
Here is a stopmotion test where you can see my flatmate picking a mug, and you can see the arc the arm is following when she is bringing the mug to her lips.
(If you also look at the boucing plasticine ball, you can see that it is following an arc).
The same thing could be applied to a thrown object.
Here is a test that shows how it would look when following an arc (You ca comare it to the streched and squashed bouncing-there was no arc to help the animation, so it looks less realistic).