Completely destroying a hard drive is hard. For the average person to destroy it, where another cannot come along and get some of the data is nearly impossible, or at least not feasible. So, those who are afraid of people getting at their data, wipe it. Or rewrite to it to push away most people. The amount of effort put in is the amount of deterence one wants. As one person told me, one cannot keep the dedicated away, but one can at least deter everyone else.
Compact discs are another matter, they can be easily destroyed. At first i tried burning them. That is, to hold it over a match or a candle. While that did work, it took a while to be noticeable. And then there's the writing on the other side. That was hard to get off even with burning. Last night, however, i found another method. It's a two-step method. The first is the fun thing of putting it in the microwave for a few seconds. Nice show, bad smell, but five seconds later the CD is toast. Step two, is then to scratch off the data. The writing, the label, and the data layer just flake away with ease. Perhaps for the truly paranoid the flakes should be burned, but it is seemingly doubtful that that is really needed. My only wonder is, if there is any residue left on the plastic disc.
No, there is no purpose. But pretending there is one make it a lot more fun.