Most of those flashes are simply from cameras left in full auto, especially point-and-shoots or casual users who don’t change flash settings. From the stands, the flash is far too weak to illuminate the athletes or field, so it usually does little or nothing for the actual subject.

Modern cameras can still produce an acceptable image because they raise ISO, use a wider aperture, or otherwise expose for the ambient light, even if the flash fires. In older cameras, this often led to poor results, such as a bright foreground and an out-of-focus nearby object instead of the distant action.

As for gear, spectators often can’t bring the ideal setup anyway. Large telephoto lenses, tripods, and sometimes even certain interchangeable-lens cameras may be restricted or forbidden unless you have special permission.

There’s also occasionally a visual-show aspect: at some ceremonies, flashes from the crowd have even been encouraged because the mass of flashing lights looks dramatic on television or in the stadium.