It’s better not to completely discharge (as a flashlight battery would do, drain it to zero)
A discharge function should (I don’t know exactly about any of these) take the charge down then recharge — to “condition” the cell, which is discussed many places.
Much discussed many places, e.g.
Batteries are the power. You will very soon want to be able to quantify and test batteries, capacity mainly, and do so in a controlled manner. A basic charger will not do that.
Discharging allows you to determine how much charge is in a battery. So you can work out the full capacity, and test how well the cell stores energy over time. Cells wear out, so knowing the capacity is useful. And manufacturers are not always reliable when it comes to specs.
You can cycle (charge-discharge) NiXX occasionally to keep them fresh. This is most useful for older non-LSD NiMh, especially if they’ve not been used much or in a long time.
Lithium should not routinely be cycled, that just contributes to wear and does nothing for performance. Checking them for capacity is an occasional task, or if something about the battery is of concern.
I had mine now for over a year and its a great charger. It can charge nearly every size of LiIon and NiMH cells. I believe i got it on sale for $ 35 bucks. More info can be found here on it: > Review: Opus BT-C3100 ("dreamcharger!")