My take...glasses do several things...cycling classes often do some of them better
1. Already mentioned, but some allow you to look forward with your head pointed down.
2. UV, glare protection...all sunglasses do this.
3. Bug and debris protection...especially important the faster you go, or in the woods, or at night, or on dirt/ recently chipsealed, or dirty roads.
4. Aerodynamics...not really a big deal unless you are a racer worried about differences with f seconds, but I'm sure the pros choose glasses that are more aero.
5. Versatility...some cycling glasses come with interchangeable lenses for bright sun, dawn, dusk, and night riding. I just got some of these, and love them