it's so "Simple" that you completely misunderstood what they are asking for
They want people to be able to do limited trials of their Applications before buying. This is important because people who are on the fense about certain applications might not buy the application if they can't try it first to find out if it's any good, OR if it fills their need.
Right now, you have to pay up front for an app, test it within a few minutes, and then if it's NOT a good App, or an App that doesn't solve the persons need, they have to go through the refund process.
Free Trials of programs has been a thing since the very early days fo computing and is a big method developers have to get word out on their programs. Apple seems to be very alone in their desire to prevent free trials.