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

Just invert the bits so that 1's are written to all 0's and 0's are written to all 1's then zero it all out that should get rid of any variable in the magnetic field and only takes 2 passes.