Why most shredding software recommends several passes?
Remember, I'm an old school HDD guy, but the multiple writes were an attempt to fully saturate the space between the tracks and make sure there is no residual data streams possible to be read between the tracks. When a track is written the electromagnetic field exceeds the narrow scope of the disk head so track data is written outside the range of the actual write head itself. What this means is the data written between the tracks which may or may not be eliminated in a single pass.
Think of it as kids coloring outside the lines and you basically have to clean out what's colored outside the lines by coloring over it in black ;)
I thought the newer technologies almost eliminated this problem in the 90s as the tracks had to become very narrow and precise to allow more tracks to be written closer together.