I don’t use iPhone/iPad apps on my Mac today, but I can see a few cases where I might.
For example, I have some accounts with a bank whose business banking website is frankly clunky and archaic. On the other hand, its iOS/iPad apps are simple and efficient. While the website is much more comprehensive in terms of business capabilities, it is much easier to perform my most common workflows through the apps. It may be worth it for me to install one of the apps on my Mac.
Of course, that would be true for any iDevice app that has no direct equivalent on the Mac. For example, I note that Authy recently discontinued its Mac app. I don’t know if Authy’s iOS app works well on a Mac, but if so, perhaps it would be helpful to some users.