Here's my list of "maybe somedays" that I'd love to have all run off a single machine:
Hash cracking. Red teaming isn't my career yet, but it would be nice if I had the tools ready when I get to that milestone
locally served "Cloud" gaming. I'm tired of being limited to a single desktop when I could be playing skyrim on my phone, but I hate supporting *aaS models—I want to own my cake and eat it too.
VM server. Basically turn everything else into a thin client. Also, what @ursakhiin said. If I ever want to do realistic training, and not just stick to hackthebox indefinitely, I'm going to need to mimic a full network's worth of computers with multiple VLANs. Or have multiple different OSes emulated to do all kinds of pentesting.
Finally start those Mastodon/Matrix/Lemmy/every other federated app instances that I've been right around the corner from hosting for ages
media server
Websites and web-apps, even if only locally served. Possibly have copies of wikipedia and archive.org and other highly usefulness-to-power-consumption ratio sites for when I eventually go off grid
maybe email... maybe. I hear it's more of a headache than it's worth, though, so maybe not
home IoT server. Handling all the functionalities so I don't have to stream security cam footage to some random company's untrustworthy server across however many hops along the way
and probably a few other ideas i've had over the years that I can't think of at the moment.
Could I accomplish all this on a couple powerful towers and a half dozen smaller/cheaper/more power efficient devices? Certainly, but this reduces cables, network overhead, and weird edge case problems having that many devices on a single-maintainer network causes. Instead of dealing with updating, upgrading, and hardening a dozen or more devices, this would give me a single point of failure that I can build resentments against whenver it has a hiccup.