Look at the circuit shown. Between the batteries you measure a current that is only half as large as the current flowing through the load resistor.
The battery provides the same voltage, however only half of the current. With the same capacity, this also means that the battery lasts twice as long before it runs out. (Actually, a little shorter, since the current through the load resistor is higher because of lower internal resistance.)
In everyday life you can find only a few applications of batteries in parallel circuits. But more on that later.
Up to this point you assumed the internal resistance of the battery to be a fixed unit. But this is unfortunately not true...