Clip-on polarised sunglasses can have an anti-glare coating which can prevent the sun from reflecting on the lens. It is less expensive to buy many different colours of clip-on frames than different pairs of sunglasses.
Clip-Ons are undoubtedly functional, if you need prescription glasses, you won’t have to get prescription lenses done twice. You can have eyeglasses and sunglasses with you always. Frames are now made with high quality, more advanced materials, and therefore you won’t have to face any “lenses smashing on the floor” problems if you drop them by accident.