Hi there. You are correct on both counts. The 'grain screen' is there to simply add a little 'noise' to the final finish as opposed to a more noticeable texture overlay (like brushwork or the canvas showing through).

My grain screens are usually set up like this. Take any image, blur it out and desaturate to get some interesting B&W shapes. This just gives a little bit more visual interest. You can also start with a simple black layer or paint in your own shapes or gradients. Add noise via the PS Filters. Set the layer to Overlay or multiply depending on your needs and drop the opacity down to around 10-20%. You can also duplicate and sharpen this layer(to make the effect more noticeable, then erase some areas where you want less focus.