The glasses are ultimately how Apple will move beyond the iPhone which has hit its peak. Our Apple Watches will be at the centre of our digital lives, our most personal devices, capturing data full time and always there at the raise of a wrist for any question or request.
Screen size won't be important for many interactions since essential data can be presented on a smaller Watch screen and UI can happen through conversation like it happens between two people rather than tapping away at a screen.
Three things have to happen for the Watch to replace the iPhone. Condensed processing power in a tiny package, a leap ahead for Siri into a natural conversational user interface and glasses that replace the iPhone's screen for visual interactions.
The first is already happening. The Watch is already more powerful than is needed to run watchOS without delays though more power will be needed to run Glasses from the Watch. On the second, Apple is investing heavily into ML, AI and Siri itself. The final piece, a large screen for contextual visual computing, will come with the glasses. Rather than looking at a screen in our hands, we'll look out into the real world and see information overlaid on objects, people and places. Want to browse through your photos, read through your emails or sit back and watch a video? Look at a blank wall or a table top and that's your large screen, much larger than any iPhone that you could fit in your pocket.
This Watch launch is going to be an absolutely massive shift, as bigger or bigger than what the iPhone was for personal computing.
The iPhone isn't going away but it'll lose its role as our primary devices. The computer on our wrists will take over that role. I can see Apple even de-emphasizing the iPhone brand and shifting it to being a smaller iPad that we can fit in a pocket. Not to read too much into Apple removing the iPhone brand from this year's devices, but that could be the beginning of that shift.