You can think of the extra capacity as dead weight, or useless if not being used, but it still has benefits even when not being used to the limits. Larger battery pack means you can stay at higher voltage longer. It means you can draw power down faster and charge up faster with less stress on the cells. It means you can limit charging to 30-70% SoC and have less degradation than if you had to utilize more of the capacity. It also means as your car ages and you lose 10, 15, 20% capacity, your vehicle will still enjoy having range to spare.