Rhetoric is communication designed to have a persuasive effect on its audience.
Here are some common rhetorical reasons for making diagrams and maps:
Your ontology already exists.
Ontology always exists, but the one you have today may be messy or nonsensical. If you were trying to understand the ontology of your grocery store, your map might look like this at first:
By asking your customers for words they think about within a grocery store, your map could grow to reflect overlapping and related terms.
If you were choosing words for the aisle and department signs or the website, this exercise would help you along.
The way you organize things says a lot about you.
Classifying a tomato as a vegetable says something about what you know about your customers and your grocery store. You would classify things differently if you were working on a textbook for horticulture students, right?
How you choose to classify and organize things reflects your intent, but it can also reflect your worldview, culture, experience, or privilege.
Those same choices affect how people using your taxonomy understand what you share with them.
Taxonomies serve as a set of instructions for people interacting with our work.
Taxonomy is one of the strongest tools of rhetoric we have. The key to strong rhetoric is using language, rules and structures that your audience can easily understand and use.