Just some of my ideas about the situation:
A standard cheap electronic lighting transformer will only start to work correctly once the minimum loading is reached, before that level is reached the transformer might only flicker or not operate at all.
This is something to do with the two sides of the transformer needing to reach (a term which escapes me at the moment I am thinking hysteresis but this is not right, think synchronous amplitudes) and then it will start working.
LEDs as a component require a minimum voltage to operate and also require a constant current because without any control they will continue to conduct higher and higher current until they burn out.
There are various electronic controls in an LED lamp (variable depending on manufacturer).
A 240V LED will generally have a current limiting resistor and a rectifier, usually with smoothing capacitors at minimum so it is internally controlled. (the smoothing capacitors are why they can flicker under induced voltage because the capacitor charges and then, once charged enough, discharges to the LED and it briefly lights)
A 12V LED will usually rely on the transformer controls to operate correctly as 12V dc is much closer to the supply they actually need.
Using standard lighting transformers supplying 12V the current is not closely controlled and the LEDs can be overloaded and eventually fail. Normally if the load is sufficient for the transformer to operate then they will work but the lamp lifetime is shortened considerably.
LED drivers are, as Drew says of two types, one providing a closely controlled output voltage, where the current will be controlled by the lamp, and one providing a closely controlled current so that the LEDs are operated at the optimum current and have a long life time.
It is particularly important where the LEDs are in a series string that the current is evenly distributed so that one LED does not burn out causing a cascade failure.
Overall using the appropriate transformer for the lamp type allows them to operate correctly and to maximum life (and costs loads to keep changing transformer, which then fail and cause more cost.)
Oh but 240V, no problems!