Biting insects are more than an irritation: midges and mosquitoes can make time outdoors unbearable, and ticks can carry Lyme disease. A good repellent keeps them off your skin in the first place, which is both more comfortable and safer than treating dozens of bites afterwards. In a prolonged outdoor situation, that protection genuinely matters.
It's light, cheap and lasts for hours per application, so a bottle in a pack or first-aid kit covers a real, everyday outdoor hazard. Fewer bites also means less scratching and lower risk of the small skin infections that broken bites can cause.