Full coverage MTB helmets extend low on your temples, rear skull, and nape—giving you the most protection you can get without going to a full-face design. They're built for the high-consequence crashes that happen in enduro racing, aggressive trail riding, and bike park sessions where you're hitting steep, technical terrain at speed.
The trade-off is weight and ventilation. These are the heaviest non-full-face helmets and run the warmest, so they can feel stifling on long climbs or hot days. If you're doing milder cross-country trails or casual singletrack, an extended rear coverage helmet will still protect your occipital bone while breathing noticeably better.
Go full coverage MTB if you're regularly riding steep, rough terrain where crashes are more likely and more severe. For general trail riding or gravel, that level of protection is probably overkill.