When you start developing an application, the first thing you do in VS.NET is creating a new project and selecting its type: Windows Forms application, Class Library etc. However, it may happen that later in the development you want to change the project's output type. For example, you could have developed a standard Windows Application - compiled as an EXE assembly - but then you want to compile it as a Class Library DLL assembly, because you don't want to make the application directly runnable, but need to run it only from another application, through reflection. It was easier to start developing it as a Windows Form, because you could execute and test it directly, but when the application is finished, you can compile it as a Class Library DLL simply by clicking Project | Properties
, and selecting Class Library from the Output type
This is also very useful if you use VB.NET standard edition, that doesn't has a Class Library project type, but allow you to compile class libraries by using this trick.