Multiple reasons. Most battery life is assuming low system load. Under high load, you can easily drain a battery in less than 8 hours. When you are traveling,

Multiple reasons. Most battery life is assuming low system load. Under high load, you can easily drain a battery in less than 8 hours. When you are traveling, the battery life of your laptop becomes a limitation you have to deal with. You may forget to charge your laptop. For some trips, you can skip carrying a power brick and use a phone fast charger over night instead. Battery charge gets slower the closer it gets to full. If 60% will last 8 hours, you can easily get away with this.