My use case would be that I add vehicle details as a subitem to a contact.
Then, based on the vehicle details, we create a proposal (that probably will go to another board) and after acceptance a project item on a project board will be created.
Currently putting the vehicle details in a seperate board, but as a new user I find it a bit of a hassle to get things linked the way I would like. Still got a lot to learn to get all boards linked correctly.