If you want to publish a version of an app immediately (or before) the new system ships out, you have to work on Beta.
Let's consider you are the developer of an interesting app with lots of paying users. Now consider you are waiting for the stable version. Now, when the new stable system version ships out, your users will install it immediately (it's automatic). And your app starts crashing, or it is displaying things wrong. You try to fix the problems but you are seeing the new APIs for the first time so it takes you one week + 2 weeks of Apple approval (which always takes the most time after a new system version is released because everybody is releasing new app versions). That means your app is crashing for 3 weeks. You have lost most of your users to competition, you are out of business.
Is this a good enough reason for you? Of course, there are also other reasons - people want to try out new features but mostly you want to fix your apps for the new system.