Safety wire is stainless steel wire that is normally used on race bikes (and aircraft) to serve as a last line of safety against nuts and bolts backing out due to vibration primarily. I'm sure google will have a wealth of information.
Special safety wire pliers are used to place the desired amount of tension on the wire when tightening it. They can double as wire cutters and as a fair replacement for needle nose/standard pliers.
I carry them on longer trips in case I need to try and fasten two broken pieces together (an alternative to duct tape) or create a new pivoting hinge with the wire, if that makes sense.
Like many things it is a bit of a long shot that it will be used, but since the pliers serve triple duty as wire cutters, normal pliers and safety wire pliers I see little reason not to include it in a kit. Several feet of the wound wire consumes negligible space.
Also, if you don't like to/refuse to wear boots when riding, [in a pinch] you could wrap the wire around your lower pant legs, to prevent them from catching on the foot pegs, when putting your foot down to slow, stop or in a quick response/fall type situation.
I just broke a rib going only about 2 mph. Bike was turning, out of gas [preparing to run it out, completely, with stab-il-ize, for end of riding season storage] & it lurched forward, quickly & UNexpectidly. Bike landed right on top of me because my left pant leg [open] bottom caught the foot peg, & down we went. I basically broke my bike's fall, & my left rib at the same time. Now, I will [since I refuse to wear any boots] tie my open/loose pant legs on my pants with a strip of cut up bluejean extra cloth strip, secure them with rubber bands or whatever suits me at the time. Even a couple pieces of wire will do the trick to secure those floppy pant legs. I will also, not wear shoes with shoe strings from now on, since I nearly went down a time or two before, when my long shoe strings [briefly] caught my foot peg. I just bought a pair of deck shoes to ride with. No strings, no pant legs, ...
... no problem
My bike thanked me