visual studio 2010 - In which cases does it make sense to have multiple projects in one solution? -

I was wondering, why do some people want to make multiple projects in one solution, if they can easily create a new project Solution for

What are the best practices / logic in deciding which setup is better?

Multiple projects are very common in a solution. It is also very rare to capture a complete program in a project can be done. A simple example is a unit test you usually want those people in your project, but the only solution is that they are easily accessible.