I imagine a manifold as like a map of the earth with coordinates or latitudes or longitudes. In the case of spacetime, manifolds composed the nodes, points, spaces and time without any matter. It's like the graph of the surface of things in graphics program without any details. Spacetime is like a manifold because it is not really solid but just composed of coordinates. I understand spacetime as just coordinates and general covariance proves it is not a thing. Hence something that is not a thing is better called a manifold with coordinates than a solid object.