Do you have any examples of these? Lattices and gradient structures are cool, but the problem is it's difficult to actually make these structures. 3D printers can make these structures, but they only accept data in B-Rep. Not to mention the 3D printers that can make these structures have closed controllers and run with proprietary software. The AMF file format can actually represent these things[0], but no one uses AMF.
Another problem is how does a human actually design these structures? 3D gradients are going to be hard to visualize much less work with. In addition, why should we have a human design gradients in these structures in the first place? Why not have a computer do this?
Gradient solids sound like they'd be useful for things like bone lattice printing. Some parts of a bone could make use of having greater density than others. eg to optimise weight, flexibility, strength, biomass holding area, in different areas.
They allow you to represent lattices and gradient structures that are essentially impossible to do with B-Rep systems.