Probably because you didn't change the shading rate of the objects in the scene.

Though it depends on the texture.  "Noisy" or highly detailed textures tend to benefit most from lowering the shading rate to really low levels.

Another thing to try is to set the max texture size to about the same size as your render size.  It's explained in the link I posted.  This can help the image look sharper, without slowing down your render times as lowering the shading rate does.