Why does the last pass of a disk certify write only zeros?

If you have ever had to recover a file that you’ve accidentally erased, you know that it’s much easier to locate the files you need if all the unused space on the disk is filled with zeros. Even if you’ve never done this yourself, and have sent a failed disk to a data recovery service, you can imagine that looking for blocks of existing data will be easier if they are surrounded by zeros—it’s like highlighting text in a page so it stands out; the zeros are like the un-highlighted text.
SoftRAID helps makes it easier to recover files by filling disks with zeros during the last pass of certifying a disk.