I use quite a few of my old machines for numerous productive tasks.
Probably the biggest one is storing and ORGANIZING all of my images / photography and music collection on my XP machines. I just can't do it (as easily) on Vista/7's UI or anything newer - to0 much clutter and certain things that used to take only a click or two on 2k/XP take numerous clicks and annoyances on Vista/7/8/10. Haven't even tried 11 yet, but I'm sure I won't like it. I like single screen, a taskbar that doesn't group stuff (buttons) for me in a tiny thumbnail that I have to click and then look through to find what I need... and... well, the list is pretty large. A NO DISTRACTIONS. On XP, I work efficiently and quickly.
Most of my XP machines have Office 2003 and Photoshop 7.0 on them. Still do a good deal of typing on them. Got an Office2k3 license when in college through that MS program (forget what it was called now). And PS 7.0 is way back from high school. At the time (early 2000's), the school was changing it out to Photoshop CS (1), so our teacher offered us a copy of the old volume licensed CD, since it would not be used anymore. Not sure where that falls in terms of legality, but it was the early 2000's and no one really cared back then either. 😉 Still use that PS7 copy for all of my image editing / retouching. It loads fast and is pretty efficient on an old PC. Sure newer versions of PS probably have added a ton of features now that can simplify some of the things we had to do manually on older versions. For a pro that is getting paid by the hour, that might matter. For me, not so much. And the fact that I don't have to pay a yearly subscription is really the best part. Same with the older versions of Office. I also have 2k7 and use that on my Win7 machines... and all of my family's PCs that I maintain, since 2k3 tends to not always like some features in the newer 2k7 format.
I also have a number of PII/P3 laptops that are not very good for Win98 gaming, but are otherwise great for audio-related stuff - mainly signal generation (the PII Dell Latitudes have really really powerful line outs) and occasionally using the line in for AF oscilloscope/spectrum analyzer via. VisualAnalyzer. So whenever I'm dabbling with a repaired or home-brew amplifier, I always use one of these for testing. The P3 laptops contain my collection of PDF datasheets as well so I can look up common parameters of common parts while working on something on my workbench - even if I don't have internet or just don't want to go online (helps with not getting distracted too.)
Then there's a number of machines that were too new 10-15 years ago when I found them to be considered retro, yet too old for modern stuff. So some of these I have relegated for various other tasks. For example, one of them I use solely for testing/zeroing IDE and SCSI HDDs (50-pin, 68-pin, and SCA). Another one, my partially-faulty AsRock 939Dual-SATA2, I use for retro video card testing, since it has both PCI-E and AGP. If it was working right, I could use it for testing PCI cards too... but unfortunately it's not - either the Southbridge BGA or the Southbridge has internal intermittent fault, making certain Southbridge-connected devices to have issues. The onboard audio is one of these and was the first to bite the dust. The SATA controller also corrupts data frequently, so cannot use SATA HDDs on this board anymore (not reliably anyways). The LAN is A-ok - corrupts bits, but recovers most of the time. Occasionally it does crash the system, though. Same with the USB ports. The PCI slots are the same too - a lot of my PCI video cards that work in other systems will sometimes show really weird artifacts on this PC. But even with all of those issues, I have still used this board to test no less than 100 video cards over the course of 8+ years. Then again, after the board issues appeared and I noticed the NB and SB run really hot, I added a fan to constantly keep these cooled when the system is in use, so I think that is what held the board from progressing any further with more failures.
And lastly... well, one could say I am using ALL of my retro systems for productive tasks, since the newest one is with a 4th gen i-series intel CPU (i7-4770s... and that's the "modern gaming" PC. 🤣 Everything else I have is considerably older. In fact, most of my "daily" PCs are Core 2 -era or P4 -era based. The early C2D's and especially the P4s are really struggling online... though if only used for admin -type stuff (e.g. pay bills and etc.), they are still tolerable. Alternatively said, they are just about an equal match, speed-wise, for my wireless connection, which gets terribly slow as soon as I move a few yards away from the router... which is the norm here, since I live in an old soviet condo with very thick all-concrete walls. And the fact that there are so many apartments around, each with it's own router (or two, if bridged), it's total chaos in the 2.4 and 5G spectrum, I'm sure. Been meaning to put wired ethernet to all of my rooms... but with those thick concrete walls, probably understandable why I keep stalling with that.