Second Curtain

[neuroanatomist]

Blinks. Basically, the time between the pre-flash for eTTL and a first-curtain flash isn't significant, but with second-curtain flash, the delay from pre-flash is enough that the reflexive blink of a subject will be illuminated by the flash, rather than being part of the ambient. Put another way, if you shoot people looking at the camera with second-curtain sync, you'll get a much higher percentage of shots with people caught with their eyes closed in mid-blink.

There might be other reasons, too, but that's the one that comes to mind...