Why would you want to though? I can understand the retro market because there is software like games that either won't run very well, or won't run at all on modern hardware. I'm in the market for a 'powerful' machine circa 2003-2005 for that exact reason.
When it comes to machines made in 2015? I'm not sure there's a lot you can run on those machines that you couldn't on modern hardware, apart from Windows 11.
I guess you could use them for things like media servers, but it would have to be phenomenally cheap, as in cheaper than cheap modern hardware.
Personally, my rule is a 10 year gap is old, a 20 year gap is retro.
I use a 2013 Lenovo X1 Carbon with a damaged screen as a plex server thanks to Elementary OS. It would’ve been e-waste years ago in different hands. Frankly it’s a shame people don’t realize how easy it is to boot something like elementary and set up a Plex server. You can be up and running in about 45 minutes and you never have to touch a terminal command.
There are tons of uses for old hardware. You just don’t know them. Pretty much any old computer can be turned into a video game emulation station.