If you ever have to read a password from your phone, either reading it to someone you're on the phone with, or reading it to type into a form, it can be incredibly frustrating not to be able to tell the difference between a zero and a capital O, or a one and a lower-case l. And, of course, there's always the tendency of your brain to stop working and leave you saying "That's M as in Mancy...", or mix up an ampersand with an at-sign.
And so, this app, where you can paste your password in, and have it give you clear, unambiguous, appropriately capitalized NATO phonetic letters and sanely-named symbols.
All of this is done within the app, it requires absolutely no network communication, so it's not sending your password anywhere, because that would be bad.