The app library is automatic. It auto organizes the apps into categories; which are mostly correct. Along with this is the ability to hide home screens you don’t want to see, and add widgets to your home screens. They work together to allow you to make a cleaner setup for your most important apps and and info, while still having a way of finding the remaining apps; in a way that is ‘hopefully’ less painful than searching through multiple home screens and folders to find that one app you almost never use.
Also, the library screen is headed with a search box to make it easier to find those apps that you at least remember the name of. You could do this before with the normal search, but it includes a lot of other stuff like websites, documents, apps you might want from the App Store, etc. The app library only searches for apps you have on your phone.
You can, of course, ignore the new features and continue using your iPhone as before. If you only have a few app, that might even be the best way to do it. If you are like me and tend to accumulate apps for obscure reasons and whimsical days, the App Library is good.
I’ve already re-organized. I’ll probably do some more tweaking, but I’m happy with the results so far.
For those of you confused, the App Library only exists on the iPhone. I expect iPads to get the App Library eventually, but for now we got nothing much new in the way of organizing our apps.