Energy saving is such a popular term recently since a couple years ago. It is not only related to gasoline or electricity at home but also related to wider matter, such as energy for smartphone. Although the power needed by a smartphone is quite low, you can imagine if there are billions smartphones and tablets active at the time. It is not low anymore. If a person save the energy of his/ her smartphone about fifth of the daily, it becomes an incredible saving for those billions overall.
There is an advantage you will get if you save energy. You do not need to bring your powerbank everywhere because the battery of the smartphone will be much longer lasting. You can use all features on the smartphone wisely so that at the end you save more energy.