Update modular-models.mdx

[aaguiarz]

Added more context about why the feature is useful

https://openfga.dev/pr-preview/pr-814/docs/modeling/modular-models

Description

References

Review Checklist

• [ ] I have clicked on "allow edits by maintainers".
• [ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [ ] The correct base branch is being used, if not main
• [ ] I have added tests to validate that the change in functionality is working as expected

[github-actions[bot]]

PR Preview Action v1.4.7 :---: :rocket: Deployed preview to https://openfga.github.io/openfga.dev/pr-preview/pr-814/ on branch gh-pages at 2024-08-19 13:45 UTC