Is there a reason why a resource should belong to a specific project?

GCP Projects are a way to easily manage access and billing of GCP resources. 

Resources in a project can only be changed/removed by users with access to that project, so giving each team or app dev group their own project is a good way to manage access control. Also, your GCP bill can be broken up by project, so various projects can easily be charged to the correct department/cost center. 

So most of the time, deciding what project a resource should go in is a matter of who should be allowed to access it, and who is paying for it. I hope this helps!