How is sandboxing directly relevant to the usefulness of an application? It can be relevant to some applications: generally applications that are made more use

How is sandboxing directly relevant to the usefulness of an application? It can be relevant to some applications: generally applications that are made more useful by being able to infer things about the global state of your OS. E.g., an unarchiver that wants to default to the current directory instead of asking you each time. But certainly not all. And not even most.