On some firmware versions it did make a difference, and it also depends on the camera / firmware combination.
For example: My 5231R-ZE always worked fine with WDR at night (black and white) on a older firmware version. It made the white and black pop out more. On the latest firmware version i have installed, WDR does not make any difference at all. I think Dahua has disabled it, although the function is still available.
On the other hand, on my 49225T-HN, WDR works fine with the latest firmware. It makes a difference, although increasing or decreasing the value does not make a big difference. At Dahua they play much with settings in the firmware haha.