A mountain bike helmet provides noticeably more protection than a road helmet for off-road riding. The extended rear coverage safeguards the back of your head during rearward falls, and the added temple coverage shields areas that road helmets leave exposed. That extra coverage matters on trails where you're more likely to fall at awkward angles onto rocks, roots, and uneven ground.
Mountain bike helmets also come with a visor, which does more than block sun—it shields your face from low branches and helps deflect debris kicked up by your front wheel. Road helmets skip visors because they can catch wind at higher speeds.
The trade-off is slightly more weight and less aerodynamic shaping, but that's rarely a concern on trails where speeds are lower and terrain is unpredictable. If you're doing cross-country, trail, or all-mountain riding, a mountain bike helmet is the right call for the coverage you need.