A CMOS battery is used to keep power to the CMOS which is a computer chip on the motherboard that stores settings information on the computer and also keeps the computers clocks settings correct if the computer loses electrical power.
If your settings are not staying where they should be or your computers clock needs to be reset every time you lose electrical power then you CMOS battery is likely bad and needs replaced.
CMOS batteries usually last 5 to 7 years so if it's been 7 years it's likely you need a new CMOS battery.