This etching takes place on the smooth surface of a plate that came out of a mechanical press. Surface area is gained by this etching. With additional surface area to conduct electrical ions, current passes more readily through the battery. High rate discharges, without excessive voltage loss, are made possible. Charge current is also accepted at greater values without overheating.
Old batteries can also benefit from the break-in process.
Batteries that haven't been cycle for a few months may show resistance to high rate charges. Don't throw the batteries away until you tried to rejunvenate them with a break-in process. Just like a car engine should be used moderately during the break-in period, so should a battery. High rate discharges and charges should be avoided. The first step to the break-in process is a good charge, including a short period of overcharge.