I have had several things over the course of my career that:

1) I was (temporarily) the only one still at the company who knew why it was there

2) I only knew myself because I had reverse engineered it, because the person who put it there had left the company

Now, some of those things had indeed become unnecessary over time (and thus were removed). Some of them, however, have been important (and thus were documented). In aggregate, it's been well worth the effort to do that reverse engineering to classify things properly.