Extended rear coverage provides noticeably deeper protection at the back of your head, covering the occipital bone and upper neck area that standard road helmets leave more exposed. For gravel and light trail riding, this extra coverage is worth considering because crashes on mixed terrain often involve backwards falls where the rear of your head is vulnerable.
The trade-off is that extended rear helmets are slightly heavier and may feel a bit warmer since there's less airflow at the rear. However, modern designs have gotten much better at maintaining ventilation even with the added coverage.
If you're sticking to smooth paved roads, standard road coverage is generally sufficient. But for any riding where you're tackling uneven surfaces, loose gravel, or technical sections, the added protection of extended rear coverage gives you a meaningful safety upgrade with only minor comfort trade-offs.