Disclosure: I build Schemity, a desktop ERD tool - this post is from our blog and uses it for the examples.
TL;DR: The meaning of a schema - what tables are for, who owns them, why a grouping exists - usually lives in heads, spreadsheets, or wikis that drift out of date. Schemity attaches markdown descriptions to entities, legends, and context views, shown behind a file icon on the diagram itself, so the data dictionary lives inside the ERD and stays where the schema is.
You can write what a table, a group of tables, or an entire view of your schema means directly into the diagram: Schemity stores a markdown description on entities, legends, and context views, one click away behind a file icon on the canvas. The data dictionary stops being a second document you maintain next to the ERD - it becomes part of the ERD.
That matters because the second document is where schema knowledge goes to die.
Where does the meaning of your schema actually live?