When you use VS or VCE to develop applications, you do so by creating solutions. A solution, in VS and VCE terms, is more than just an application. Solutions contain projects, which might be Windows Forms projects, Web Form projects, and so on. Because solutions can contain multiple projects, you can group together related code in one place, even if it will eventually compile to multiple assemblies in various places on your hard disk.
This is very useful because it enables you to work on shared code(which might be placed in the GA) at the same time as applications that uses this code. Debugging code is a lot easier when only one development environment is used, because you can step through instructions in multiple code modules.