Here’s a scenario when you’d want a battery tester:
Let’s say your hearing aid suddenly stops working. You are trying to clean it and are unsure if wax is blocking the sound coming out (as you can only see so far into the receiver tube). If it’s still not working, and you just grabbed a fresh battery - then test the battery!
Some times batteries or whole packs of batteries are sent to wholesalers dead! It would be nice to know if you’re hearing aid is broken or if you just had a bad batch of batteries.