I wrote the docs section you refer to :)
They are meant to be narrow and to the point, not didactic, not least because the codepath leading up to an option and following that option depend on internal evaluations and other options.
> why would anyone want to do that?
Because the input has weird or unreliable DTS. One would have to check code history to see why an option was initially added. Many of ffmpeg options are there to deal with edge cases or weird inputs.
> -fflags +genpts+igndts -use_wallclock_as_timestamps 1
In theory, igndts is unsetting wallclock dts set by the last option. genpts should have no effect, again due to the third option.