What are the project GUIDs in a Visual Studio solution file used for?

Question

I have multiple projects in a single Visual Studio (2008) solution.
I just discovered that each of these projects uses a same GUID, so in the solution file it looks like

Answer 1

I will often copy a VS project file, rename it and search and replace all occurrences of the project name to create a new project with same settings. This is often quicker than creating a new project and then ensuring all the settings are correct. I ignore the GUID. VS will change the GUID as soon as I save the project after adding it to solution.

I do a lot of DLL/C/C++ development and I have deviated from the standard project settings extensively over the years (particularly in regard to optimization, code generation etc) and I find the copy technique a quicker, surer way of setting up a project.