Why A battery behaves like an ideal voltage source in series with its internal resistance, so terminal voltage = EMF minus I times R(internal). The smaller that internal resistance, the less the voltage sags and the less energy is wasted heating the cell when heavy current is drawn. That means a low-resistance cell can deliver large currents, which is why starting and high-power transmit applications call for them.
Watch out Rapid recharge is the closest trap, since charge current also passes through the internal resistance, but the pool credits the discharge side; internal resistance does not set cell voltage (chemistry does) or by itself determine service life.
Low internal R means low voltage sag under load, so it can dump big current.