My answer on a different post: App or extension? - #4 by cho-m
In brief,
- some current features (e.g. Windows Hello support on browser extension) will require Desktop app.
- there are future feature requests (Feature Requests - Bitwarden Community Forums) that may require Desktop app to support.
- otherwise, I mainly use Desktop App as an alternative UI that is closer to Bitwarden Web Vault (Bitwarden Web vault). As daki28 mentioned, the Desktop App (or Web Vault) is better for doing large modifications.