Sometimes for good reason (you are on a classified project, etc). Sometimes for bad reason (we don't want anyone to be able to prove anything).
The latter is something bad lawyers suggest. It basically never works to get the company off the hook. I'm not going to say "never never", but most of the time it leads to a situation where it's easy to convince a jury you are malicious and evil.
Even without a jury, i will simply put your peers who don't do that on the stand and make you look like you have a non-standard policy of hiding things.
Or i'll find the human who asked why. Because in any company of any size, people eventually ask each other why they are being asked to do this, and someone will say the wrong thing ("You know why we do this"). Most of them will not lie for the company because they don't buy it either.
etc
This sort of hiding strategy goes wrong in many many ways, and only goes "right" in a remarkably small set of circumstances.