The charger varies the output as required by the battery, to bring it up to full state of charge.
The 12 Volt supply just gives you a fixed voltage up to its current rating, the current which flows will be determined by the load.
I did drive my fuel pump on the parts 1998;to empty the fuel tank with a charger, but for testing stuff it’s better to have a fixed voltage supply. Paradoxically, the fancier digital chargers are more likely to give you trouble since they usually have failsafe circuits that shut them off if they see a load they don’t “ like”