Model Composer is particularly useful for algorithm exploration, design prototyping, and model analysis. When these are the goals, you can use the tool to flesh out an algorithm in order to get a feel for the design problems that are likely to be faced, and perhaps to estimate the cost and performance of an implementation in hardware. The work is preparatory, and there is little need to translate the design into hardware.
In this setting, you assemble key portions of the design without worrying about fine points or detailed implementation. Simulink blocks and MATLAB M-code provide stimuli for simulations, and for analyzing results. Resource estimation gives a rough idea of the cost of the design in hardware. Experiments using hardware generation can suggest the hardware speeds that are possible.
Once a promising approach has been identified, the design can be fleshed out. Model Composer allows refinements to be done in steps, so some portions of the design can be made ready for implementation in hardware, while others remain high-level and abstract. Model Composer's facilities for hardware co-simulation are particularly useful when portions of a design are being refined.