3D presets are basic 3D shapes that come ready-made for immediate use. No need to draw an outline or begin with a photograph; primitives can be created on the fly and pressed into service straight away.
Having said that, some primitives have more sophistication and more wide-reaching possibilities, such as the bottle composed of three parts or the texture-wrappable cube. As with so much in Photoshop, there’s more to primitives than at first meets the eye.
Start by making a new, empty layer. Then, in the 3D panel, set the source as that layer using Selected Layer(s) and make sure the Mesh from Preset radio button is checked.