There are really two questions in your post.
First of all, regarding nesting. Nesting makes sense when it conveys a “part-of” structural hierarchy. For example, a stile is part of a door and you would most likely want to be able to manipulate the entire door as one object. Nesting the stile component inside a door component handles this.
However, you should avoid getting too carried away with nesting because it can make a model hard to work with. The Outliner helps a lot, particularly since its performance and features have been improved in recent versions of SketchUp. But having to constantly drill down through multiple layers of part-of hierarchy can both slow you down and create confusion when you aren’t editing the group you thought, but instead have either its next outer one or its next inner one open.
Next about solids. Think of a solids as the models of single real-life objects. So, in my previous example, the stile would surely be milled from a single piece of stock. It should be a solid. The door is an assembly of several solid parts, and should be modeled as such - but it is not a solid itself.
If you are planning to 3D print a model, solids are essential because 3D printers can’t generate impossible objects, and most non-solid shapes are physically impossible. For example, a “surface border” is a gap in the surface of an object. One can pass through this gap from the open air around the object to the material within the object without passing through its surface. That’s physically impossible, and the printer can’t create it!
If you aren’t going to 3D print, the main value of solids is checking whether your model has flaws, some of which may be hard to spot. For example, there might be a tiny surface border hole due to a drawing error. Or there might be edges or faces that stick out from the surface. These are both physically impossible structures: the edge has neither width nor thickness, and a face has no thickness. In a model they might be strays that should be erased. Similarly, interior faces usually serve no useful purpose and should be erased. If you see that a group or component is listed as solid in Entity Info, you can be sure it has a physically realistic structure.