[CMake] (newbie Q) What does command PROJECT do?

[Jun Sun]

Initially I forgot to set PROJECT name, but it seems working just fine.

Later on I thought it might be equivalent to the "project" concept
in a whole Visual solution. So I started to put a bunch of "PROJECT"
commands for every logical build unit. That seems to work too.

Now I read the document. It say:

Set a name for the entire project.

Well, so should I just have one PROJECT command in the top-level 
CMakeLists.txt file? And exactly what does it do anyway? When does
it matter?

Thanks.

Jun