you need the battery in place. it keeps the bios powered when pc is turned off. without it all your bios settings are lost each time you turn the pc off.
the battery is like $2 for a 2 pack so not like there is a big reason to not just replace it if it is dead.
an old one is fine but of course would be that much closer to dying on you that much sooner. they usually last up to 10 years in my experience.