bout 10 years ago, the norm was to, from time to time, drain lithium batteries to minimum and so do a full cycle, this is something my father told me but I actually don’t know the reasoning.

Early rechargeable batteries such as nickel-cadmium and nickel-metal-hydride would develop "memory". For example if you made a habit of always recharging the batteries once they hit 50%, the battery would think "I guess they don't need the rest of the capacity, I'll throw it in the trash" and you ended up with a battery with half it's original capacity. So it became good practice to occasionally discharge them completely before recharging. Sort of a 'use it or lose it' scenario. Now lithium batteries do not have this issue but it took people a long time to break the habit.

I believe it's because software managing batteries needs to calibrate voltages to understand how charged a battery is. Fully power cycling allows, for example, your phone to understand what 100, 0 and everything in-between.