SkaveRat is addressing my original question: I'm asking if there is an advantageous reason for this phenomenon. You seem to suggest it's a spandrel at best, and fair enough, that could be the answer. It probably is a spandrel, I also believe that.

However spandrels usually don't reduce future chances or reproduction, and this one clearly does, so I was asking perhaps there is an advantage to this feature (not a spandrel then). Or at least an explanation for its existence from a genetic perspective, ie. the genes triggering the self destructing behavior are also the same ones responsible for a major survivability feature.

The reason behind spandrels existing can sometimes be explained other than "random", as it happens with the human chin for example - apparently someone figured out it's physically impossible for a chin not to appear if you are deforming maxillary bones to flatten into a face.

So far here nobody knows for sure about the octopus, and I gather it's because science doesn't yet have a consensus on the matter. But everyone has been quick to assure me it's just random and that there isn't anything else to it without any scientific backing.