Motion paths are simultaneously the one of the best and worst things about animation in powerPoint. Best because they are so powerful in the effects that you can achieve and worst because it is so easy to get distracted into spending far too long trying (days) to work out how to achieve a particular effect.
One property of motion paths is the endless animation loop, such as the bouncing ball shown below
Reasonably stepwise instructions on how create an endless loop animation.
Hey-presto, instant and endlessly bouncing ball. Click on the demo to show two bouncing balls.
To create additional balls in the powerpoint (available here), simply duplicate the one that you have made and adjust the motion path. You can totally alter the motion path by entering "Edit Point" mode (red diamond) and right-clicking the path again.... the context menu that pops up this time includes an "Add Point" option. Add points. Lots of extra points were added (and then dragged around) to alter the path of the green ball. BTW, if you increase the length of a path, it is usually a good idea to slow it down by increasing the duration of the animation event.
Of course, "Why", as in why would you ever want to do this, is a perfectly valid question and finding a use for endless loops is something that is entirely up to you. I generally use then to represent molecules, as in the animation below, where I am trying to explain how organic anions are secreted by renal epithelial cells. If nothing else, it is completely hypnotic and available for download here.
More fun with infinite loops here.
These animations are complex, but not necessarily difficult to achieve. They all use the same basic technique over and over again. The only tricky bit is making sure that the appropriate elements appear and disappear at the right moment. Take a look at the "Animation Pane" for these animations... they look a lot more complicted at first glance than they really are.