This can be handy if you happen to have an external keyboard connected and would like to use it to type in different languages. However, there are a couple of iOS wrinkles you need to work through to get this to work properly.
FIRST, regardless of what layouts you can access in Typefinity, the ones available on the hardware keyboards are only those listed under the system Settings in General | Keyboard | Hardware Keyboard. (Note, you must have your keyboard connected for this option to show.)
However, you can't edit these directly. Instead the entries there follow the system software keyboards you have set up under General | Keyboard | Keyboards. So you need to add the languages you want there.
THEN, there is a second step: you have to go into each Hardware Keyboard for which you've added a system software keyboard, and make sure it is NOT set to "Automatic".
Finally, once this is done and you have successfully switched between the different layouts on the hardware keyboard using Typefinity, you can then usually remove the system software keyboards if desired. The hardware layouts should still remain. (But in the case they disappear you'll have to repeat this process.)
Yes, this is confusing. :-}