There is no purpose for discharging the battery other than to cycle them in order to find out their capacity. Simply putting a load on the battery and letting it drain down serves no positive purpose for the battery, and in fact it can damage a battery. If a battery is discharged below 1.1v in any cell it risks what is called cell reversal. When this happens the cell is shot and is not longer usable for flying. If you are going to discharge a battery it needs to be done by something that can monitor the voltage and stop it when the voltage gets down that low.

Years ago batteries got a benefit from cycling them regularly (this always seems to get a lot of discussion too as many will argue the point), but with current battery chemistries there is no benefit from cycling the batteries. The reason a battery should be cycled now it to determine it's capacity, nothing more. When a pack is new it should be cycled 3-5 times to determine (it can take several cycles for a new pack before it settles down and is "broken in", so to speak) it's initial capacity. Then periodically the pack should be cycled to see what it's current capacity is, and then compared to it's initial capacity. For me I will take a pack out of service when the pack is 80% of it's initial capacity.

Ken