Simulink is a rich modeling language for any system ,but why integration between this simulink and SysML (other modeling lang) is required?

Simulink itself is a powerful tool ,it can design and simulate any type of system.then why Integration between simulink and other modeling language like SysML supported by IBM RATIONAL RHAPSODY tool is needed?. In which areas(which type of systems) simulink is going to fail.

please give some information on this, that is useful for me. Thank you