I use data modeling tools because they allow me to document important information about the database other than what "fits" in a database. Meta data like privacy/security/sensitivity concerns, stewardship, governance, etc.

That may go beyond what some need in documenting a database, but those things are important to the business and helping them manage their data.

Formal tools also help me in managing data that is stored in more than one database/instance/server. This has never been more true than in our packaged application world.