The CMOS battery is not there to provide power to the computer when it is in operation, it's there to maintain a small amount of power to the CMOS when the computer is powered off and unplugged. The primary function of this is to keep the clock running even when the computer is turned off. Without the CMOS battery, every time you turned on the computer you would need to reset the clock.
On older systems the CMOS battery also provided the small amount of charge required to maintain the nonvolatile BIOS memory, which remembered BIOS settings between reboots. On modern systems this information is typically stored in flash memory and does not require a charge to be maintained.
