I bet double flares are mandatory on automotive applications. "Your brake flares provide a leak-proof connection between the hoses and fittings, without any need for a sealant. All brake lines need to be double flared, due to the high hydraulic pressure. If your brakes leak or the hoses crack, it could prove to be fatal. Single flared lines are suitable for low-pressure lines in other applications, but not your vehicle’s brakes. For example, your vehicle manufacturer may use them in fuel and coolant lines, since they’re cheaper than double flared lines. It also wouldn’t be quite as dangerous for those lines to break. They’re not under the same amount of pressure. Double flared lines, on the other hand, help reduce friction at the flares, which can cause cracking or leaks."