What is the purpose of wearing sunglasses while cycling?

Wearing sunglasses can help protect your eyes.

Sight is one of your five main senses that you should value greatly. You can choose to wear it based on:

-the weather condition you will be facing.
-the environment you will be at.
-the activity you will be engaged in.

Your eyes will be uncomfortable if any of the following particles of nature are constantly hurting your eyes:

-sand
-dust
-wind
-snow

Wearing sunglasses can protect your eyes from flying debris hitting your eyes while you are out on the road, especially while riding on a bike or bicycle, that does not provide much protection. Corneal injuries can occur even when small particles hit your eyes at high speed because they can become deeply lodged in them. Sunglasses can also reduce the chances of pollutants ending up on the surface of your eyes which can trigger inflammation. You will be stressed out and not be able to enjoy the activities for the day if you have:

-blurred vision.
-swelling and itchy eyes.
-eye strain.

These may cause headaches and make you feel tired sooner.

If you are often out under the sun, consider wearing sunglasses with lenses that provide UV protection as prolonged exposure to UV rays can increase the risk of:

-cataracts.
-age-related macular degeneration (AMD).
-eye cancer.

Do you know that there are helmets for cyclists with sun shields attached? They can be retracted or removed when they are not required.  Most later models have magnetic studs to hold the shields in place on the helmet. 

The sun shields are made of plastic. So, if your sun shield is attached with magnetic studs, it is best that you remove it from the helmet first before you put it on or take it off your head. This is to prevent the sun shield from accidentally slipping off the helmet and landing on the ground which can cause scratches.  

This is a multisport helmet with sunshield. 

As the shield curves around the face when the helmet is worn, it a better option over getting a pair of regular sunglasses.  It works better at protecting the cyclist from getting hurt by dust particles blown into the eyes. 

This model is supplied with two retractable sun shields in different shades of colours.

This model also include a visor for days when you do not require the sun shield.

I like this model for kids with full face shield attached.

Though such helmets are not SIRIM approved for motorcyclists, some of my customers buy them for their children to wear when they take them out for rides on their motorcycles. This is because it is difficult to find helmets in smaller size that are well fitted for their children to wear. Most children are attracted by the cute images printed on the helmets and will consider wearing them.

I think it is also useful for children who are too small to wear masks. The full face shield will give them some protection against getting infected by Covid-19 virus while out in public.