In previous versions of iOS, all of your apps are on one of the home screens. In iOS 14 they can be on a home screen or not, but they're always in the App Library. When you upgrade, your existing app icons are left on the same home screens they were originally on and you can choose to leave them where they are, or remove them from the screen but leave them in the App Library. When you install new apps, you can have their icons placed on a home screen plus in the App Library, or just in the App Library. You can also choose to hide any of your screens except the Today screen. If you've got a lot of home screens but you only want to use the App Library, it can be easier to hide the screens than to go through all your icons removing them one at a time.
As for why it's switched on, it probably shouldn't be. But new OS versions are often buggy, and work differently for some people, depending on their setup. An OS update (which is what this was) can be more buggy than an app software update. Many people are complaining of very heavy hits to their battery with iOS 14, especially those who also have Apple Watch.
What many people seem to be doing, from what I've seen on Reddit and elsewhere, is setting up a minimal Today screen, adding a primary home screen with their most-used widgets, plus a secondary home screen with their most-used apps, and maybe a few folders for lesser-used apps. For some, it's faster to pull down on the home screen, tap the mic icon on the keyboard and say the name of the app they want. For others, "Hey, Siri! Open Mail" is quicker.