When I'd talk at Ascend about how Visual Studio auto-deploy of SQLCLR assemblies not only catalogs the assemblies and the PDB files to SQL Server 2005 but also the source files, folks would always ask "why the source files"? Today I found out; I'm surprised it escaped me for so long.
In Visual Studio Server Explorer, if you've deployed the source files and PDB, you can right-click on any T-SQL or SQLCLR stored procedure, UDF, etc and and debug it, source code and all, whether or not you have the actual project files in-hand. It will just get them from inside SQL Server. Pretty neat…so that's what its there for.
There is a pretty hefty (but *completely* understandable and necessary) price to pay for admission to this feature, however. You must be sysadmin in SQL Server (CONTROL SERVER) to do this. Understandable because you're going to:
a. stop all managed threads on the server
b. be able to snoop around
So…that's why. And how.