These are both a safety feature and a handy widget. The Safety thing they do is that they allow the fender stays to automatically release from the fork dropouts when a paper cup or branch or some object in the road gets sucked-up in between the front tire and fender, preventing the wheel from otherwise stopping immediately and causing you to fly over the bars.

The practical and Handy thing is that they allow you to take your fenders off faster and more easily. Although only intended as a Safety thing for the fork, there's no rule against using them on the rear for easy fender removal.

Popping off the fender stays sans tools makes taking the rear wheel out of rear facing track style dropouts (like on the SimpleOne) tons easier. The SKS fenders and hardware kits already include clips but if you don't need to buy everything else we now offer the clips by themselves. And being plastic they do break from time to time.

Sold individually

If you don't use them, this could happen: