Batteries wear over time. For lead acid based batteries this is because of the corrosion on the grids of the positive electrodes. Lithium based batteries wear because of lithium ions diminishing on the cathode and lodging on the anode.

Either way, the batteries will perform worse under load because the internal resistance increases as a larger current passes through the resistor in the battery. Older and worn batteries have more resistance in the battery which will only be obvious when putting the battery under load.

Obviously we want the battery to work even in the most demanding situations, therefore they have to work high load conditions.