Why create your own package:
Organisation, If you put everything in the default package, after a few years you would have several thousand classes in the default package and finding anything would be just about impossible. It would make naming a new class difficult - you would need to check if the name was already used.
Reuse by others If everyone put everything in the default package, how would you distinguish between your layout class and johns layout class and peters layout class
I would try and give your package a sensible name - helps If you need to refer back to the code later.
i.e.
shop.swing.exercise
rather than
exercise1
also see
Why shouldn't we use the (default)src package?