Imagine you wrote sensitive corporate details on a whiteboard. Once you leave the room, you simply wipe the details from the board. I can still read most of it by looking at the whiteboard under an angle, or by using advanced techniques to lift minute traces of marker residue from the board. On the other hand, if you wrote random gibberish on the board after wiping it, it would be a lot harder for me to read any of the corporate details.
This analogy for overwriting a hard-disk with zeroes vs random data is why many people believe it is better to overwrite your hard disk with random 0's and 1's, and more than once while you're at it.
Simply overwriting all data with zeroes will certainly stop 99.9% of the population from reading your sensitive data. Overwriting it with a predictable (as computers are inheritly non-random) pattern of 0's and 1's will make that a bit harder still. Using a cryptographically secure pattern will make that still harder. And repeating this process will raise the bar even more.
But due to diminishing returns, I believe one pass of pseudorandom 0's and 1's is more than enough. And if its not, you'd better bring your hard disk to a secure data destruction company instead of increasing drive wear and wasting CPU cycles and time. Hard disks are cheap.