Howard had to buy some spare CR2032 batteries for my Snark Tuner the other day, they seem pretty good, and at that price for 10 they're a bargain. It would probably cost me that for two batteries at my local supermarket. My Snark does eat batteries if I forget to turn it off.
I have mains power supplies for all my pedals as well, but sometimes just having a quick noodle it's handy to just run on batteries.
I'm assuming most guitars with active pickups run on the 9v batteries, so would be pretty handy for that purpose.