The purpose of a diagram in an informational text is to <em>summarize information in a visual way.</em>
An informational text is part of the non fiction category, it aims to share knowledge about the <em>natural or social world</em>. For that instance diagrams come in handy when trying to understand a process description, like photosynthesis.
A diagram can help us understand and remember ideas because it's a <em>graphic aid that sumarizes information</em> using visual resources.
