What is it for? For order!
So that all transport requests belongs to certain logical group(project). Also it allows to prohibit the release of requests for certain projects.
- Transaction SPRO_ADMIN.
Create several projects:
Go to the project > Transport Requests > CTS project status switch:
Select All configured transport targets:
Change Status Switch for Project:
After that no one can release a transport requests attached to this project:
- Transaction SE03
Go to Display/Change Requests Attributes:
Change SAP_CTS_PROJECT attibutes: 
Change to Mandatory:
After that no one can create a transport requests without specifying a project.