Think of it as how you select your house/city in The Sims 2 or SimCity. Each planet is it's own "household/city," this lets you create and play with more then just one creature and galaxy, and while you may interact with your own creations, they are all separate and independent of each other.
For example:
You have two planets, two creatures, Creature A & Creature B.
Playing as Creature A, you discover Creature B's home planet, it looks just like the planet you have on your other file, and you decide to blow it up. When you load up Creature B, it's home planet is still there, just as you left it.