For me, it’s being able to use a computer while lecturing. Taking attendance, drawing on slides and opening diagrams is awesome. Also, reading and annotating PDFs while writing on my Macbook is something I need to do all the time.
Unfortunately, 90% of my work cannot be achieved on iOS. My job involves using either Java IDEs or databases administration tools and neither work on iOS. Some web apps are possible on mobile safari but it’s painfully slow to use compared to a Mac, and both the web apps I use daily at two different universities do not work at all on mobile.
I think the main reason so many web apps are incompatible is simply the user base. I have 300 students this year and not one uses an iPad, so the developers of the web apps are not going to focus on iOS.
I use text replacement instead of Text Expander and this works throughout iOS. I’ve never needed the more advanced features of TE so I’m happy in this respect.