Speaking for myself, it does one thing incredibly well: email. It probably seems absurd to imagine it would do that thing better than other places where I can

Speaking for myself, it does one thing incredibly well: email. It probably seems absurd to imagine it would do that thing better than other places where I can do that thing, but it's probably 20 or 30% better at email than my other options. And I do a LOT of email. I keep an iPad Pro (with keyboard) to the left, and when I want to do email, I look left and do it there. That keeps me focused on my main windows. I do think that it would be possible for someone to write an email client that is as efficient as the iOS Outlook app is, and maybe it already exists for non-Outlook, but in my current corporate environment the iPad pro is a lifesaver.