The visual studio that you get is a shell called Business Intelligence Development studio and it allows you to create Analysis Services, Integration Services, and Reporting Services projects. Also it allows you to deploy those projects to your SQL Server. If you already have Visual Studio installed, it will add those project types to it.
With the Development Studio you can add .Net projects such as C# or VB to your Analysis Services or Reporting Services projects to create custom components for them. There are additional benefits, but you should just do a search for Business Intelligence Development Studio to see what else can be done.
I just noticed that you were asking about what happens if you uninstall Visual Studio 2008. There should be a listing under your installed Programs called Visual Studio Shell or something of that nature, which is the Business Intelligence Development Studio and is independent of the main Visual Studio 2008 install. Both of them will allow you to create the Ms SQl Projects that I listed above. When you uninstall Visual Studio 2008 it won't uninstall this application and vice verse. If you do accidentally delete it, you can always install it again using your MS SQL CDs. There are instructions on the web to do so.