If I have an emergency out on the trail, I assume it'll be the first bit of gear pressed into service. If I need to get someone's attention while in a strange city or if I'm out for a jog, it's always ready to go. It also serves as security blanket, reminding me that I can take some sort of action in a crisis. Luckily, I've never actually had to use it (well, other than during Purim services one morning, where I didn't have a functioning gragger).
So, you can imagine my sheer delight when I read Legal Nomad's travel advice and found her recommending a whistle as an essential pieces of kit. She links to a story where she outlines a number of times a whistle saved her butt. Like the time she fended off a pack of wild monkeys:
Gasping for breath and watching several of the bolder monkeys creep toward me, teeth bared, my eye caught the bright orange whistle of my safety whistle. Figuring I had nothing to lose, I inhaled deeply and pushed out the air as hard as I could, scaring those monkeys with a sound they’d likely never heard before. They yelped, they scattered. I kept moving as quickly as possible, looking behind me in a bit of a panic. But the monkeys had all disappeared.