I think a good 30 second answer to this question is to explain that the sahara desert used to feature lush growth. Some people suggest that it became a desert due to over-intensive agriculture. Permaculture is about reversing this process.
10,000 years ago, people learned to plant their favorite foods close by so they wouldn't have to go far away and find them. Only these plants always seemed to need care while the stuff in the wild didn't. Permaculture is (IMOO) about understanding why and attempting to work more symbiotically with nature so that these plants can be healthier and more productive with less water, fertilizer and effort.