Most "MTB" helmets come with visors, most "road" helmets don't.

I like a visor to keep glare down, even though I'm almost exclusively a road rider. Helps for light showers, but a hard rain will come through.

I've got one helmet without a visor, which is great for winter riding with a helmet cover. (It's also darker commuting in the winter, so there's less glare from the sun!)