Helmets without visors are built for road cycling, time trials, and triathlon, where aerodynamics and low weight are top priorities. A clean, visor-free profile slices through the air more efficiently, which matters at speed or during long efforts. That's why most road and aero helmets skip the brim entirely.
The tradeoff is no built-in shielding from sun, rain, or overhead glare. Road riders handle this by wearing a cycling cap underneath the helmet—the small brim provides shade without the aero penalty of a visor. Paired with sunglasses, you get plenty of protection for most conditions.
If you're riding road, gravel, or racing, a no-visor helmet is the standard and usually the best choice. Visors become important for mountain biking, where low branches and trail debris are real concerns. Don't assume you need a visor just for sun—a cap and sunglasses typically work better for road-style riding anyway.