Internal resistance is not just theoretical. Ohm’s Law states that E=IR, or the voltage is the product of the resistance and the current. Georg Ohm needed to define resistance, so he picked an easy value.
So let’s say you are drawing 200 Amps from a 12.6 volt battery. If the internal resistance is 20 mOhm, then the voltage drop within the battery is 4 volts - meaning you will get only 8.6 volts at the terminals.
Now you need to decide if, at 200 A, 8.6 V is ‘good’ or ‘bad’. Note that it is not necessarily bad, since 200 A is an extreme value not often seen in a 150 second match.
Also note that this is the technical explanation for a brown-out.