Database diagrams enable you to modify database objects without updating the database until you explicitly save the changes you have made. If you modify a table and remove it from a diagram without saving it or close the diagram without saving it, the table continues to exist in memory in its modified state until you close any other open diagrams.
During an editing session, you can experiment with different object definitions to see how a proposed modification will affect the database. When you complete the modifications, you can either save the diagram and thereby update the database to match the diagram, or you can discard the diagram and leave the underlying database unchanged.