How to optimise battery usage on modern electronics?
There is always some internal resistance in a battery. All other things being kept constant, the larger the battery, the less internal resistance, so less energy is wasted inside a larger battery due to internal heating and energy loss.

This is particularly important in high-drain devices such as phones and laptops, where batteries are discharged in just a few hours. By having the power bank in parallel, the load is shared and batteries are being drained over a longer time.

For example, a once-cell penlight with an incandescent lamp might burn for an hour. Two cells, one replacing the other, would then last two hours. But two cells in parallel might last 2.2 hours, because of the lower internal resistance and decreased energy waste in the cells.