Dear Sueilla,
I do not know which one of my posts is not clear for you.
Any how, here we are discussing about the new Methods of App's Distribution which has been put in force by Apple without any prior notification.
Since the new method was not notified, after upgrading to iOS 9.0 and facing problem with syncing to iTunes, many people including me thought that something must have gone wrong during upgrading to iOS 9.0 or there is a bug which prevents a clean updating to iTunes. Now it has been turned out that the Sync Issue is due to the new App Distribution Method using by Apple.
This new method may have some consequences for some users, as you can see many of them here on this thread, those who for instance may have a limited internet band width. The new method may cause some problems for such people.
Obviously we were not talking about compatibility of old apps with iOS 9.0. I hope it is now clear for you.
Have a nice time.