I've been given a mid-2013 MacBook Air i5 4GB for use when I go do some research work and learning.
Going into its 11th year, with 850 battery cycles, the machine still works great for what I need. 4-5 hours of battery life, lightweight, easy to transport, and good port selection. I mainly use web browser, PDFs, some math/engineering software like MATLAB, nothing crazy just stuff on the go. Sometimes it's just to back up a camera card on the go, or make edits to a latex document.
HOWEVER, macOS is pretty unbearable on it. Not only does it run very slowly, to the point of just being tolerable with enough patience, but the non-retina display makes text look fuzzy enough you'd swear it's time to get glasses, since Apple removed sub-pixel text rendering. So if I only used macOS, this machine would definitely feel its age much more than it does, because...
Thankfully, Windows and Linux (Mint Xfce in this case) run beautifully on it with its measly 4GB of RAM (wish the original buyer bought 8GB as that would easily breathe more life into it, but alas Apple's criminal upgrade prices and stingey base RAM...). I lose about an hour of battery life (so typically 3-4 hours) but it's fast enough to not feel burdened but my workload and text looks perfectly sharp again.
As to the original question, why use such an old machine? Because it does what I need. It reduces unnecessary e-waste. The OS is fully up-to-date if I use Windows or Linux, and not far behind with Big Sur. The real question is why would I buy a new machine just to do things I already do?
The main reason older computers start to feel old is because newer software bogs them down. It doesn't have to, especially considering how little it changes these days. It's funny how well software from 2013 runs on it. I can still use my fully paid-for copy of Photoshop CS3 (2009) on the Windows boot and it works as well now as it did then. Of course, macOS updates break old software so again, you have to use newer versions that don't run so well on it.
Last year, a family PC from 2013 was going to be replaced for being unbearably slow (and it was truly unbearably -- probably 3-4 minutes just to boot, a full minute to open web browser). It's an HP with i7 4690, 12 GB of RAM and some low-end nvidia dGPU, but it had a hard disk. I replaced the HDD with an SSD for like $50, did a fresh install of Windows, and boom -- faster than ever. Maybe 12 seconds to boot, 1-2 seconds to open software. For their use (web browser, email, photos, office) it's about as fast as a new computer, despite being over 10 years old now.
Personally, I'd rather donate $1000 to one of my local animal rescues currently saving a bunch of precious animals than upgrade before I actually need to.