Academic Statement of Purpose vs NSF Personal Statement
There will be many common features, but the NSF has many highly stylized requirements, including "broader impact" and such, which would seem stilted or hypocritical outside that venue. NSF has a page limit, rigid formatting requirements, many other formal requirements, none of which matter in a statement of purpose. You have room to be more sincere, more genuine, explain more fully in your grad school application's statement of purpose. Also, unlike the NSF, where there is some compulsion to pretend that you already know what you're doing and will start research immediately, grad school applications (e.g., in mathematics) can easily and reasonably acknowledge that it might be premature to be thinking about PhD-level research prior to grad school.

(I don't necessarily hold a grudge against NSF over some of this, because they have to "make the sale" to Congress, in terms Congress-people can understand and find compelling, in order to get the money...)