Ever look inside an old phone? It has a tiny battery in addition to the removable main battery.

1. It causes the phone to irreversibly corrode if dropped into water.

2. It maintains the clock when you switch batteries.

3. Probably some other nefarious purpose. The most benign might be storing encryption keys to prevent phone cloning. Storing keys in battery backed ram is one of the most secure ways to do it. See Capcom Kabuki and DigiCipher II.

Ever look inside an old phone? It has a tiny battery in addition to the removable main battery.

1. It causes the phone to irreversibly corrode if dropped into water.

2. It maintains the clock when you switch batteries.

3. Probably some other nefarious purpose. The most benign might be storing encryption keys to prevent phone cloning. Storing keys in battery backed ram is one of the most secure ways to do it. See Kabuki and DigiCipher II.