I would say PhD proposal might be used to see initial direction and focus of your current and potential future research. And it might also be used to reflect how a candidate student experiences in academic writing. As it is used for initial direction, once you enter your PhD program, you might research different things according to a project given to you.
So it depends on the type of PhD program you are applying to. In most the cases, if a PhD application is submitted through the university or for scholarships, PhD proposal are one of their requirements.
Ismail M, PhD candidate, Computer Engineering, Kwangwoon Univ, Seoul
Source: Quora Digest