There's no question that apps have completely changed the use of those cell phones we keep in our pockets. We do things from on our phones which make our lives richer, easier and quicker. For example, having the a map of the whole world in your pocket, ordering a pizza, sharing pictures with everyone and just staying connected in general.
With all these apps has come an economy of its own, creating thousands of jobs and creating whole new streams of revenue. Look at Angry Birds, a simple game where you throw birds at pigs. However some reports I've seen have said that Rovio, the people behind Angry Birds, make $1 million a month from Android alone - that's purely from advertising. Remember, that doesn't include their toys or iOS apps.