Pfffft... a lot of online services (banks, HMRC) have the double-login thing where they send you a verification text before you can get in. They won't email it, they insist on texting. Some landlines can report a text msg to you, some can't, and I might not be at home when they want to send the msg. Or else you can wait a week for a letter to arrive inthe post with the access code...
Some websites don't even work now as websites -- the onlin service only works with an app that you need on a phone (or tablet). Like my fitbit, or the app for managing my bike lights that runs ONLY on an Android system.
The Ebay App is the easiest way I have found to get pictures attached to listings. I can find workarounds, they take much longer than the phone app. So does bank transfer of money.
Strava: I don't want a Garmin when the app can do to track my run (don't even need to use Data to do that)
Twitter works best with mobile phone format (designed for the small screen).
I can login to my bank accounts much faster using the phone apps (fingerprint) than I can online (could get a completely different laptop with fingerprint sensor, I guess).
I'll be travelling in Netherlands soon & very pleased I'll have their NS/trainline equivalent app to use to get around, there are like 5 changes on the journey, each way. I can quickly look at all the booking information (ferry, AirBnB, conference location, insurance details) on the phone, much smaller item than a sheef of papers would be.
Phone is Not a necessity but so many things would be extra headaches to get done without.