Some LED driver manufacturers claim that LEDs automatically share the current equally and such external current mirror circuits are unnecessary. This is not necessarily true. There is always an imbalance, unless the combined forward voltages of the LED strings are identical.
lf, say, two parallel strings are mounted on a common heat sink, then if one string draws more current than the other, it will run brighter and hotter. The heat sink temperature will slowly rise, thus causing the Vf of the second string to fall and causing it to also try and draw more current. In theory, the two strings should then balance out their currents because of the thermal feedback. In practice, this effect can be measured, although that is not enough to guarantee accurate current balancing.
Furthermore, if the two strings are in fact two separate LED lamps, there will be no thermal compensation feedback. The lamp with the lowest combined Vf will draw the most current, will run the hottest and the Vf will still fall further. This will make the imbalance worse and can lead to thermal runaway and LED failure.
Today, the current mirror solution is rarely used. The cost of accurate LED drivers has fallen so low that for high grade lighting it is better to run each string from its own current controller. For low grade lighting, any current imbalance and the resulting reduction in lifetime are acceptable for cost reasons.