Allows you to use AA batteries in devices which require C batteries. I have many AA batteries, and no C batteries, nor any way to recharge C batteries. Therefore I'd rather put my AAs into the devices which need C.

Just slot the battery inside and insert. This model has a bevelled edge so that the batteries go in smoothly and you don't need to navigate the springy bit on the negative terminal.

Also, the bevel allows for very easy removal of batteries from the device when it's time to replace them.

Tested with GP Greencell Rechargables, and my favourite Panasonic Eneloop / Eneloop Pro batteries, for a snug fit.