Is the purpose of an internal resistor in a practical voltage source to set the maximum possible current that can be drawn?
No, it is not there purposely to reduce current, it is a parasitic effect.

If you look at a battery, it does not contain a separate resistor, it is just the structure and chemistry of the battery that causes internal series resistance.

Or a transformer, it also does not contain a separate resistance, it is the resistance of the copper coil itself.