Why overwrite a file more than once to securely delete all traces of a file?

Question

Erasing programs such as Eraser recommend overwriting data maybe 36 times.

As I understand it all data is stored on a hard drive as 1s or 0s.

If an overwrite

Answer 1

There are "disk repair" type applications and services that can still read data off a hard drive even after it's been formatted, so simply overwriting with random 1s and 0s one time isn't sufficient if you really need to securely erase something.

I would say that for the average user, this is more than sufficient, but if you are in a high-security environment (government, military, etc.) then you need a much higher level of "delete" that can pretty effectively guarantee that no data will be recoverable from the drive.