If you don't reason from first principles you can only come up with ideas that are slight derivations of existing ideas. If you reason from first principles you can come up with completely new ideas that have never been tried before. If you continue this process you don't have to worry about people stealing your ideas because you'll come up with so much stuff that you'll never run out of new things to try.

The one problem with reasoning from first principles is it becomes hard for anyone to judge your ideas and give you feedback before you try them. You have to be able to check yourself because most people, when introduced to a novel idea will think "Sounds like a good theory" and shrug their shoulders because they can't compare it to anything similar in their personal knowledge base.