I could be wrong but I believe the idea is that the growth mindset has measurable consequences. I.e., if you believe you can grow, you will learn/adapt more effectively to new situations and ultimately achieve more. It's an empirically verifiable claim that is either true or false, and comes with a pretty straightforward intervention (i.e., teaching people that they can grow) if true.

Of course people may also adopt it as a personal philosophy, but that's separate.