What is the "Personal Photo Letter" intended for?
As its name implies, the Personal Photo Letter, is a personal letter, that is, a letter written to a specific person, that also includes your photo along with your name. 

It is like any traditional personal letter, only it has your photo in it. 

There’s no real “intended for” situation where you must use that letter. Only when having your picture in it makes sense. I don’t know or I’m not aware of cases where using this type of letter is considered a requirement. 

Sometimes when writing a resume you might want to include a presentation letter. Putting your picture there allows the reader to associate your face with the resume or CV. Other than that, I wouldn’t include a picture of me in a personal letter.