One application where I put this constant current supply (let's shorten that to CCS) to good use is to revive old rechargeable batteries. For example, when NiCad batteries were still widely used to power cordless phones and other gadgets, sometimes the battery's internal resistance became so high that it could no longer be charged with a normal charger and had to be discarded.
This CCS can supply more than 40V at 15mA. The high voltage can force current into the battery while limiting the current to a safe value. The forced charge reforms the battery's internal chemistry and gradually reduces the internal resistance. The battery's terminal voltage actually decreases steadily until it reaches the normal voltage. It can then continue to be charged with the CCS or with its own charger.
I did this a number of times. The revived batteries may not have quite the same mAh capacity as new ones but they are quite usable. Some of my clients continued to use them for a long time.