"neccessity". The ones I wear most often are clear however. If I ride for a long time my eyes get sunburned, so the UV-blocking ability of the lenses is key. Also, the sunglasses reduce the number of bugs in the eyes (a particular problem in the evening), and I can ride at night with confidence my eyes won't get poked out by an errant tree branch.

Also they are really good for winter riding, as wind at -30C is very harsh, and glare off of snow is really bright, however this leads to a whole host of fogging/icing issues.

When its raining I use yellow lenses (great for visibility), and when it's very sunny I wear dark lenses.