There are two reasons people do this:
If your path/name to your animated sprite node changes, you'll only have to change it in one place, not throughout your code. This is of course only a benefit if you are going to access that node multiple times.
Caching the node reference in a variable could provide some minor performance benefit, although it's debatable whether the impact would be negligible. Again, only a benefit if you're accessing the node multiple times in your script.