If you want your code to run on the GPU, the complete viability of your code depend on it.

Because of the performance improvements from vectorization, and the fact that GPUs are particularly well suited to that? Or are GPUs particularly bad at branches.

it is only one of the many micro-optimization techniques you can do to take a few nanoseconds from an inner loop.

How often do a few nanoseconds in the inner loop matter?

The thing to keep in mind is that there is no such thing as “average developer”. Computing is way too diverse for it.

Looking at all the software out there, the vast majority of it is games, apps, and websites. Applications where performance is critical, such as control systems, operating systems, databases, numerical analysis, etc, are relatively rare compared to apps/etc. So statistically speaking the majority of developers must be working on the latter (which is what I mean by an "average developer"). In my experience working on apps there are exceedingly few times where micro-optimizations matter (as in things like assembly and/or branchless programming as opposed to macro-optimizations such as avoiding unnecessary looping/nesting/etc).

Edit: I can imagine it might matter a lot more for games, such as in shaders or physics calculations. I've never worked on a game so my knowledge of that kind of work is rather lacking.