So many uses. They are a convenience and not really necessary when I really drill down into it, but the convenience factor is significant.
Maps and navigation. I can change plans on the fly and still confidently navigate cities I have never been to before, with a good estimate of when I will get somewhere. Could I plan from home or use paper maps/atlases? Sure, but a smartphone is way easier and more flexible.
Communication. It simplifies keeping in touch with people and maintaining relationships. Could I call them from my home landline once a month? Sure, but a smartphone allows me to send them a quick text or a dumb meme anytime from anywhere. Smaller, more frequent communication to supplement the less frequent larger effort helps a lot with relationship maintenance.
Having a computer in my pocket any time I am away from home and my home PC.
People who eschew basic modern technology are weird. It’s just a fact. Could I move to a remote mountain cabin and throw out all electronics and let my inner, introverted, weirdo quirks run rampant? Sure. But I don’t really want that. I want to have friends and to be generally well thought of by my community. I want people to text me to invite me to go out. I want to pull up the latest artist I’m excited about to share with someone when we’re out and about. Smartphones are so ubiquitous, it is a radical statement not to have one and I simply don’t want to be a radical.