maximum flash sync speed, and the Nikon SB-900 / SB-910 speedlight
Because of the way the focal plane shutter works in DSLRs, shutter speed doesn’t affect our flash exposure … while we don’t go over maximum flash sync speed. When we go into high-speed flash sync, our flash output drops. (The linked article there explains it thoroughly.)
However, when we work with ambient light (and intend to add flash to our subject), then a change in shutter speed has an indirect effect on our flash exposure. A change in shutter speed will mean a change in aperture, and this is what affects our flash exposure (with manual flash), or our flash output (with TTL flash.) Check this article for how manual flash and TTL flash differ.
Therefore when we add flash to ambient light, then our shutter speed choice becomes important.