Its cool and fun and teaches you a skill at a low price point. There is essentially no functional reason to do this over a single high power server unless for some reason you need a bunch of ARM servers for something.
If you can distribute your workload (or it's not intensive anyway), i.e. having no single machine with a faster processor, then I think it's a good choice, cheaper to run.
Edit: I'm happy to be corrected if down-voters would like to reply... A Pi 4 draws 6-7W at full load. A former desktop PC turned server likely idles over 100W.
They're different beasts, so I'm not saying 'a Pi is better than a PC', I'm saying 'if your load could also be handled by a (bunch of) Pi(s), then all else being equal that might be better than the PC'. Of course all else is never equal, ymmv, you may already have the old PC, you may need very many Pis, so even if pennies cheaper to run it might take years and years to break even on the Pi acquisition cost.
I also like the idea of having an easy homogeneous setup, each Pi having one disk, vs. my alternative (old PC parts) being something like '4 SATA ports on this one, and 6 on that one, but one's broken, less one on each for the boot disk' - I quite like the idea of having a bunch of Pis, and one image that I apply to all of them, which can include details like /dev/sda being its only data disk. That's not really a functional advantage, just appealing for keeping it simple to come back to and remember/understand how I set it up when I need to tweak something (and really easy to add another one increasing available cpu/ram/disk).
Picture in Swedish, from graphics card review comparing average power draw for the entire system for various GPUs, idle and while benchmarking in Metro Exodus, numbers rounded to nearest multiple of 5: https://cdn.sweclockers.com/artikel/diagram/22452?key=2c6f21...
If the direct picture link doesn't work, it's under "Effektmätning" here: https://www.sweclockers.com/test/30908-amd-radeon-rx-6900-xt...
[1] https://magpi.raspberrypi.org/articles/raspberry-pi-specs-be...
[2] https://www.pidramble.com/wiki/benchmarks/power-consumption
Y'all negative folks against this seem to have very restricted use cases in mind. Not all redundant rack servers live in a 36U case in a air-conditioned internet connected server farm. There are plenty of use cases outside of this.