From the introduction:
There may come a point when you have explored the entire world, or someone has done it for you. You crave to see new horizons, to travel to places other can only dream about and to experience wonders and dangers that are delightful and dangerous, sometimes both at the same time.
You may be a powerful spell caster seeing how far you can stretch your powers or a new god-like being bored with this universe and wanting to see what you can. You may even be a scientist, exploring the multiverse and the realms beyond.
Whatever your reasons, the Plane Maker is here to help you come up with the details about various planes, from both a magical and technological point of view and somewhere in between.