It is useful for giving presentations, or for keeping your 'commonly used' queries together. It offers Source Control integration, which makes it vaguely useful. I often combine it with PowerShell, as PowerShell lets me easily run every .sql file in order on a particular server for deployment.
But I'm not an overall fan. There are too many things it doesn't do. I love SSMS, but it's not a proper development environment.
The Miscelleanous section can be handy for storing the odd Execution Plan. If I have a query that I know prefers a particular plan, it can be handy to have that plan 'on hand' for reference.
