You can check whether the fonts are installed in Settings>General>Fonts
It's to allow using custom fonts in apps that allow the user to choose fonts. Previously you could only use the fonts that were pre-installed on iOS. For example, in Pages, you can choose your custom fonts now. So I presume, rather than for changing the UI, it's more for when the user is creating documents/slideshows/illustrations etc. I've tried and it currently does work in Pages.
I suppose some apps may now offer the user a way to change the font used in the UI of the app, but I doubt many would do this, if it was possible.