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