Because the bios battery is used to retain the settings of the Date and time RTC Real Time Clock. it always have to have power to it. that is why the battery is there to keep the time and date right. And retain all the current saved settings.
Isn't it cmos battery alone is enough to save the date/time setting? I only detach the notebook battery, so that means I only use power adapter to start the notebook. All this time, there is no problem of BIOS date/time resetting itself until recently.
Unplug the main power without a Cmos battery = Bios restet every time.
Did you mean my cmos battery is missing? I haven't checked it yet though.