Often Model Composer is used to implement a portion of a larger design. For example, Model Composer is a good setting in which to implement data paths and control, but is less well suited for sophisticated external interfaces that have strict timing requirements. In this case, it may be useful to implement parts of the design using Model Composer, implement other parts outside, and then combine the parts into a working whole.
A typical approach to this flow is to create an HDL wrapper that represents the entire design, and to use the Model Composer portion as a component. The non-Model Composer portions of the design can also be components in the wrapper, or can be instantiated directly in the wrapper.