I suppose that models that are painted in the flat style have a texture problem. Yes, as 3D objects, they react to their light environment naturally (and tend to look strange in show case lighting). But the problem is that they still have a texture/surface made out of acrylic paint which has different reflective properties than the objects they're supposed to portray. The acrylic surface is not glossy enough for metal and too glossy for rougher textures. So in the end, the miniature will not react "convincingly" to a natural lighting situation. That might be part of the reason why metallic paints have been developed for miniatures back in the old days. I suppose those metallic colors aim to change the texture and make it more reflective and shiny, which makes sense if you rely on dynamic lighting in a dynamic (ever changing) natural light situation. In the volumetric approach to painting miniatures, you turn off any natural environment and paint the illusion of texture yourself. You "freeze" a particular lighting situation on your model, so to speak, a situation over which you have full control.

The whole point of a wet palette is that it should allow some water to mix with your paint, keeping it fresher, so if your wet palette isn't doing that you may have the wrong kind of paper on it? You can also add a little bit of water to thin your paint, just like you would on a regular palette, but I find it is ever so slightly less because of the wet palette. Hope that helps