The reason of people's wearing goggles under their helmet may be for the vision. Actually they could also wear the contact lenses to help them see things clearly which will make it convenient for them to wear. Now wearing the contact lenses become the fashion which may bring a lot of convenience for people in their daily life.
Helmet can protect drivers' heads in accidents while goggles can protect them from accidents when they are driving especially at a high speed. Firstly, it can prevent the impurities in the air from getting into eyes as the driver is in a bare environment( not like in a car) . We may have to blink frequently if there is something in the eyes, which can be dangerous for drivers. Secondly, the speedy wind can cause pain in eyes which can lead to tear so that may dim their vision.
After all, a clear vision can be very important for drivers when they drive. And that's why wearing goggle can be very meaningful.
Hope that can help.