Charging a LiIon battery to the highest possible voltage gives it maximum capacity, so that is the preferred setting if you intend to use your laptop as a portable computer, because it will run for a longer time.
However, higher voltage will cause the battery to age faster. So, when connected to mains, keeping the battery at 100% charge all the time is not the best. A lower state of charge puts less stress on the battery and will allow it to keep its capacity for a longer time.
The value is a compromise: 50% would be even better for battery preservation, but it is unpractical: in case you actually need to take the laptop away from a socket, runtime will be low and the battery will end up in a deeper discharge, which also ages it faster. So, 80% seems like a good choice.
Basically the end of charge voltage on a LiIon battery is always a compromise between how many years you want the battery to last, and how much runtime you want out of it.
There is another factor: at 4.2V the charger needs to proceed with caution and reduce current because this is close to the overcharge limit. So when the laptop draws pulsed current according to CPU load, the battery may go through many tiny charge-discharge cycles depending on how the motherboard is designed.
If the battery is at a lower state of charge like 80%, the charger does not have to be "careful" to avoid overcharging at all, so it can provide a lot more current without risk, which means, if it is well designed, the charger will provide for all the current the laptop uses, and the battery will not go through these constant microcycles. That's another reason it will stay in good shape for a much longer time.