Why are the AirPods such a success? After all, the iPhone could play back audio via internal speakers and wired (and even wireless) headphones were/are widely available. But the AirPods are very convenient and provide a smooth user experience.
The glasses will have a similar use case: Smartphone and "normal" monitors are widely available, but the glasses will be able to provide a way more comfortable user experience.
- Want a monitor with 2-3 meters diagonal, freely positionable without taking up physical space? Here you are!
- Want to get routing guidance directly in your field of view and additional info about that building/car/person/object over there? Here you are!
- Want to support your aging eyes with variable focus on objects of interest (a.k.a. "Zoom")? Sure - here you are!
- Want to have sunglasses with adjustable dimming levels? Well - here you are!
Vehicle highlighting in traffic (battery-electric vehicles can be dangerous for hearing-impaired persons), reduced neck strain (-> "Smombies"), hands-free video while commuting in overcrowded subways, privacy (no one can look onto your mobile phone screen anymore), individual TV programs for each family member at home without arguing about which program to watch ... - the options are endless!
If Apple does it right, they have a big, big winner on their hands! At first people will complain "I don't wear glasses and don't want to change that now" - just as people complained about wearing a watch when AppleWatch entered the market. And they will complain about the price for "just another iPhone accessory" that can't work standalone and is much more expensive than your standard glasses from the local superstore around the corner.
Eventually people will get used to it and it'll become a new trend to wear glasses again. (Very) longterm there may also be contact lenses becoming available for the die-hard refusers of glasses. And it'll be very interesting to see how the competition will react and offer cheaper, but potentially technically inferior and/or less refined products to participate from the new hype.