At least two people have come close to the answer, maybe actually did answer the question. If not clear, let's try again.

Modern test equipment is high impedance, meaning that practically no current is required to gat a voltage reading (from the suspect battery). It could read close to normal voltage. However, when a load is added (the battery is put into service) it cannot deliver the required current and the voltage drops. If the device the battery is installed in has a circuit to sense low voltage, it may (will) shut off the device.

If these are carbon (cheap type) throw them away. If they are rechargeable, recharge them. If they are alkaline, they may have some service left as flashlight batteries or emergency backup for a small radio.