In short:
writing to zero's is like getting a brand new un used hard drive. in effect
a new hard drive has millions of sectors. when data is added, these sectors change to the relevant data, and is made 'ledgible' by the computer.
erasing/formatting a drive doesn't change the data much, just makes the computer 'think' a new, clean hard drive is installed. the data is still there, but gets written over when needed (if you lose a lot of data on a drive, and you are considering data retrieval DON'T USE THE DRIVE. it'll start writing over, quickly, as soon as it uses the disk for anything, thus losing the retrievable binary)
writing it all to zeroes acually goes through every sector, like a bulldozer, making sure every bit of data is over written by a 'nothing'. multiple passes only offer further security.
warning:
It takes bloody ages. like about 2 days for a decent hard drive at 16x passes