Christchurch Mosque Shooting, page-167

Flashing light is for flash aim quick targeting .
I won't go into to much detail. It makes eyes focus instantly on human silhouette even among shadows and background.
Allowing for quick and rapid fire and targeting without using optical sights.

It's something used when assaulting or breaching buildings and tight spaces that aren't well lit, by a lot of special forces.
There is a reason he had that on.

There you go, now you got all of it well explained.
Cheers