Imagine you are texturing some kind of terrain. You apply displacement like noise or some kind of fractal, and then texture it using triplanar. It makes sense to displace first, and then texture.
Also, if the texturing was done first, and then displacement, then you could easily end up with stretched textures - which triplanar is supposed to prevent.
Last point - it does not make much sense to talk about UVWs with triplanar, as triplanar is designed to make UVWs obsolete. :)