It is supposded to be used only when doing test renders. If you move objects, change their material (mostly if these materials are reflective/refractive), change render resolution, and of course camera position, then the previously computed and stored radiosity solution will be wrong.
But you can reuse it if nothing has changed in the scene as far as indirect lighting is involved, but you want to change, for example your AA settings, or the quality boost of a spectral atmo.