What is the significance of internal resistance?(15PTS)

Internal resistance in a power supply has the disadvantage that it dissipates electrical energy, thus reducing the useful energy delivered to the exteral circuit.
However, in some siutations, internal resistance in power supply does have advantage. Its presence can pose a limit to the maximum current in the external circuit as well as in the power supply itself, thus preventing too large a current if a short-circuit situation happened.
EHT power supply usually has quite a considerable internal resistance to limit the current that it can deliver (since a high voltage together with a high current is extremely dangerous, the limitation to the max current acts as a safety feature to protect the user). Therefore, when a light bulb is connected to it, the current through the bulb is small. That's why the bulb only glow dim.
A 12V battery usually has much lower internal resistance as compared to that of a EHT power supply. Thus when the light bulb is connected to it, the current through the bulb is considerably large. This makes the bulb to glow brighter.