Thank God for that. That makes them actually useful.

Personally, I find sandboxing not as something desirable, but a symptom of a problem with mobile/web ecosystem.

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.

The majority of desktop apps that I use on OS X are Sandboxed (from a quick glance: Chrome helpers, Wunderlist, Microsoft Office, Photos, Airmail). I haven't experienced any functionality downgrade since the pre-sandboxing times.

As long as iTerm runs without sandboxing, I am happy ;).