I remember reading a story about a couple of college age adults who hiked to top of a cliff they intended to camp near over-night, and then take the return hike back down in the morning.

They get all the way to the top and decide to take their bags off, sit down, and enjoy the sunset. One guy accidentally got his foot hung up in the strap of another guy's bag. Not realizing it, when he got up, his foot flung the bag over the side of the mountain. Unfortunately, loosing all his buddies gear.

Luckily for them, it was only an inconvenience. However, something like that could easily take a turn for the worst.

The basic idea behind a small PSK (like the candy tin versions) is that it should be something small and light enough that it's always on your person. That way, if you do somehow loose your pack, you've got something. I mean, a tiny knife and a few matches is better than no knife and no fire-starting gear.

Personally, I don't carry one (as I feel my EDC is substantial enough with significant redundancy). However, I think it's a sound idea for a lot of people. Kids and teens especially. They might leave their bag behind, they might forget to take their pocketknife and fire-starting gear, they might lose their PSK out of their pocket....but the chances of all three happening are much slimmer than the chances of one or even two happening.