A primary advantage of illuminated reticles is:

Illuminated reticles provide a significant advantage in low-light conditions, enhancing the ability to see the crosshairs or aiming point in environments where natural light may be insufficient. This feature allows shooters to maintain target focus and ensure precise aiming, particularly during dawn, dusk, or in shaded areas where visibility is reduced. Better sighting in these conditions can be crucial for successful shooting, whether it's for hunting, competitive shooting, or tactical scenarios.

The other options may not showcase the primary advantage of illuminated reticles effectively. While improved accuracy can be a secondary benefit, it largely depends on the shooter's skill level and environmental factors. Reduced weight is not typically associated with illuminated reticles, as they often involve additional components like batteries and electronics, which can increase weight. Lastly, the cost-effectiveness aspect is also secondary; illuminated reticles tend to be more expensive due to the advanced technology used in their design. Hence, the advantage of better sighting in low light is the primary reason illuminated reticles are favored.