What is the purpose of the diodes on a Samsung LED TV backlight bar
The diodes are there because LEDs can't handle very much reverse voltage.
Common white LEDs have a forward voltage around 4V while the maximum reverse voltage can be as low as 5V.

Suppose the driving voltage can become negative, then the LEDs would be reverse biased, have a high impedance, and -without the diodes- no current would flow. In that case the (now negative) supply voltage may become higher than it would be in the forward direction, as it is essentially unloaded. There is no way to tell how this reverse voltage will distribute over the LEDs; One LED may take most of the voltage and fail. 

The (protection)diodes "short out" the high reverse voltages over the LEDs to protect them. I think you should not leave them out.