Why would someone ask the voltage of a battery?

Well, a 12 volt battery is made up of six 2v cells. With AGM chemistry batteries each cell is 2.14v so a 12v battery is actually 12.84 (or almost 13 volts).

When someone estimates a batteries state of charge they check the voltage and as a rough guide reduce 25% of a batteries capacity by 0.25v. So a completely flat battery is approx 11.84 volts and 50% capacity is approx 12.34 volts.

These figures aren't correct for many types of batteries like flooded (calcium, maintenance-free), or GEL but are to serve as a guide. Flat is not zero volts. A battery below 2 volts will rarely recover to fully charged unless it is brand new. Capacity can be dramatically reduced if a battery is left for a few weeks in a discharged state.