You have been misinformed. Modern batteries are not at all keen on being fully discharged, and this can significantly affect their working life. (They're not too keen on being overcharged either.) The best practice is to keep them topped up, don't let them discharge fully, but don't keep them on charge once fully charged.
No quarrel with that, except to say that the control circuitry in the battery pack should prevent over-charge.
One point, more or less on topic, is that if the batteries don't occasionally go into deep discharge the "how long left" calculations get a bit fictional.
For Apple laptops, if the accuracy of that calculation is important they suggest it's "re-calibrated" occasionally (every few months) by running the battery completely
flat (ie use it until the computer goes into hibernate mode, then leave it for a few more hours) before re-charging.
I'd imagine the same applies to PC laptops, they're much the same batteries.