This is a bit of a long explanation, but the phrase "I never understood" triggered me. Well, there is often an assumption that a smart phone is essential to everyone and everyone has one. I have not owned a smart phone since Apple's original incarnation of the iPhone. I gave that thing up in disgust for poor reception on the old cellular network, and the final straw when all my favorite apps suddenly would not upgrade or work unless I bought the next version of the phone. A problem that still exists today, and BTW in Europe Apple has been hauled across hot coals for deliberately slowing down phones so that people will buy new ones.
Speaking of triggers......
Working in tech, I can say that apps not working unless you buy the next version is almost never true. There are several issues at play here. One of those is that app developers, having more powerful CPUs and more memory to work with, write apps that require more of of the device if they are to run. So a lot of that can be put on the shoulders of app developers.
As far as "deliberately" slowing phones, Apple did this, yes, but not not "require" upgrades. Rather, to try and extend battery life. As I'd think folks in this forum would know, battery capacity shrinks over time. Apple thought that as batteries aged, putting less of a load on them might extend their useful life. Apple's error was not communicating this clearly when they did it, probably based on some hubris that no one would notice.
And the network thing is most definitely not Apple's fault. My first iPhone ran on AT&T's 3G network, which, as it turns out, they'd done a horrible job with here in the Bat Area. When I upgraded after a couple of years, I found that they had done a MUCH better job on their 4G/LTE network. Unless there is an actual problem with a specific device's hardware, connectivity issues are almost always the network provider's fault.
That all said, they are not a "need." But considering their capabilities, they make many modern day tasks efficient, easy, and often even pleasant. Which is why when I buy my Bolt, I'll use my iPhone and my Pixel XL for navigation, music, and many of the services that Chevy wants subscription money for.