It is not necessarily a matter of choice. Besides what the other commenter notes about 2FA, in some countries banks have been removing functionality from their online-banking website, and you can only do certain things in the phone app.
The most infuriating I've seen, is a bank which removed the anual tax report (which you need to do the anual income tax) from the online-banking website, requiring you to use the phone app... to download a PDF file, which you then have to transfer to the computer anyway so you can print it!
Is it a privacy or financial risk to have banking on your phone?
How is banking on a phone app more dangerous than banking via mobile or desktop websites?
The issue is the platform. Obviously there are issues with desktop platforms too, but those are easier to mitigate.
The government services also go through these ID apps, although there is a poorly supported alternative that uses USB smart card readers. I have not seen a single person actually use it, probably for a reason, though I'm planning to get one just to have a backup...