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

The United States has requirements put out regarding the erasure of sensitive information (i.e. Top Secret info) is to destroy the drive. Basically the drives were put into a machine with a huge magnet and would also physically destroy the drive for disposal. This is because there is a possibility of reading information on a drive, even being overwritten many times.