A small pocket knife is a handy tool to have, whether you think you need it everyday or not. The general idea is that it's better to have it and not need it, rather than need it and not have it.

Beyond self-defense, which is a fairly obvious use case for any "weapon," I've used my pocket knife to cut loose threads from clothing, open plastic packaging that broke a good pair of scissors, removing stickers, splinters, and even as a makeshift screwdriver (in a pinch).

You can do almost all of those with scissors, but are you going to carry a pair of scissors in your pocket? Probably not.

Beyond that, there are emergency scenarios you may never run into, such as cutting seatbelts, clothing, or stuck shoelaces -- again, better to have it and not need it...