Having a second controller is a psychological crutch in this way. It represents the eternal possibility of that friend who is just about to burst through the doorway and play video games next to you, like when you were kids. Now purely ceremonial.
Well, you'll definitely want a second controller because the PS5 controller sticks are well known for very quickly developing problems like severe stick drift. So when your first controller inevitably goes bad, you'll want that spare, either as a replacement entirely, or as a backup while you spend weeks first arguing with Sony just to honor the agreement to fix their shit (if you're lucky enough to still be under warantee) and then shipping it to them and waiting far too long for them to return it, and then only to find they did nothing and have to go through the process all over again, to finally hopefully get back a fixed (for the time being) controller. So yes, a second controller is necessary, even if you only ever play solo.
Blows my mind that Hall effect sticks are still not simply the default technology in all controllers these days. As far as I'm aware, there are zero downsides to them.
Don't know tbh, I still only have an old original PS4 I bought used. But a while back for a brief moment I looked into what it would cost to get a PS5, I read about the major problems people had with it and stick drift is a really big issue with the dualsense controller. What I've read since is that it has only gotten worse, and there are countless nightmare stories from people trying to return or replace them. Apparently there is a "pro" version of the controller that has been out since 2022 or something and it's main selling feature is that you can swap out the sticks. The sticks still have the same drift problems, mind you, they're just modular and swappable, so instead of having to buy an all new controller every year or so, you only have to pay for a new pair of sticks. But that controller alone like like $140 or something.
Perhaps not for big new Sony titles, but if you're a family / have kids you definitely want at least two as there's still lots of good couch multiplayer games out there.
If it's just for you then possibly for stick drift (rather than getting them later when they've gone up in price again) or because they still use a built-in battery instead of rechargeables, and they don't last very long by comparison (and that play time gets even lower as they age obviously).