That last maneuver isn't necessarily intended to extend the life of the battery (lithium - ion batteries don't
like full discharges), but it does help keep the battery sensor calibrated so the on - screen meter offers and accurate reflection of your battery status.
Consider that a normal car battery is rated, say, 61 A · h, then even if your alternator could poke it with 50A for a whole hour, it'll still be left uncharged; in reality, 20A charges are often called «rapid», 10A is «fast», and the normal charges are more likely to be just 6A to 8A, which means that a fully
discharged battery of a compact car will take something
like at least 8 hours to reach the
full charge!
There are some things you can do to prolong a battery's health (
like avoiding
full discharges, and keeping it away from heat and cold), but it happens to every battery eventually.