It's also a way to gauge if you can be black-mailed to leak secrets. If you are open about your dark secrets (e.g., addicted to meth), then the chances of a foreign agency using that information to turn you into an asset are lower so there is still a chance you might get that clearance. If they dig something like that up on their own (which there is a fair chance they will), then you are guaranteed denial since it means you are a higher risk to black-mail as you were not willing to divulge such information.