Fully charging the battery can cause it to age more quickly and lose capacity. Limiting charge level helps to protect it, especially when it's always connected to a power supply and don't need the battery that much.
What actually kill batteries is overcharging (which has to be done on purpose in the electronics) and heat, having internals over 60C is where the real issue is.
I think phones are the 1st ones that started overcharging just to ensure lower life expectancy of the device... and of course much better autonomy during the 1st impressions.