I'm no authority on this, but apparently the SIM card sucks some power while the phone is on. Or gets the phone to suck some, I don't know.

What I do know is that my carrier sent me a new SIM card some years back, with a note that a) it would hold more SMS messages on-board, b) it would have room for more phone numbers, and c) it would use less battery.

But that was years before 2004, so if your card is that new you probably have a "low-power" SIM (whatever that actually is, technically speaking).