Okay, it's rather obvious why a starship would have a self-destruct system. To keep enemies from obtaining technology secrets, to keep the ship from being used by someone like Khan...

But why would the Krell have set up a planetary self-destruct system? ("Forbidden Planet.") Especially since they apparently had no idea that what they were working on might turn out badly. But even if they had some subconscious sense of caution, why have it destroy the whole planet?

And why would the Earth Ship Ark have a self-destruct system? ("The Starlost," episode 7, "Circuit of Death.")

Both of those seem more like having a self-destruct mechanism on the SS Titanic. What in the world FOR?

I'm sure there are other examples, but those are the first I thought of.

Is there a good reason, something other than Because Plot/Because Drama?