Also braking is a concern. It's what is required to make low duty cycle alternative energy sources viable especially solar. Need to store the excess energy when the collector system is being irradiated Energy storage is also important for power leveling for the power companies Generating stations operate more efficiently if they run at constant output level want to shove unused energy to a storage system and recover it later at times of peak demand.
Some materials have high energy storage capacity but low rate of recovery.
Since gasoline has much higher energy density it quickly dominated the way vehicles were propelled. In fact, gasoline has one of the highest energy density storage capacities known. This is because kinetic energy only increases linerarly with Mass but goes as the square of the rotation speed. Rapidly rotating objects are subject to centrifugal forces that can rip them apart. Thus, while dense material can store more energy it is also subject to higher centrifugal force and thus fails at lower rotation speeds than low density material.