Doing it on the go via the app is much easier than using the web app through the main OS browser just because the UI is optimized. not a problem with using the web app approach, just that there isnt as much investment in it due to zeitgeist i guess.
Also since you are already using 2FA, you are already on the phone so might as well do basic operations there.
I can also look at transactions in my bed before going to bed so that is nice.
If I need to look at a support ticket or look at transactions more deeply, i still use the desktop approach.
I don't think many people would argue that there shouldn't be a mobile app, just that there should also be a website/webapp way to do it as well if you don't want to install their native app.