Take a look atthis page to understand the philosophy of Model-Based Design. Simulink is MathWorks' model based design platform. Its graphical nature is ONE of several essential reason as to why that is the case.
Thank you Swarooph for your answer. However, The Aerospace Standard DO-178/DO-331, clearly states models can be graphical or textual. The Automotive Standard ISO 26262-8, states that models are mathematical representations (so could be textual), but only describes graphical representations (block diagrams and state machines.) At MathWorks and most of our customers, Simulink (graphical) is the platform for MBD but it is common to include MATLAB (textual) blocks inside Simulink. So some level of graphical representation is needed (for architecture and data flow), but it is ok to mix textual language components and still consider it MBD.