As someone who has never done this before, what's the benefit of setting up a server rack, compared with just using my desktop? Is it purely for the fun of tinkering with hardware? Is there something that's not doable on a desktop?
You might use less power if you have separate low power devices as opposed to a single (large) server though, so that’s one consideration. Another those is what it is you hope to accomplish. If you want to test gear or hardware that you can’t virtualize, then a rack can be handy. But even things like pulling out hard drives from a running server can be simulated with a VM in some way.
But it’s easier to brag about having a rack than a really elaborate VM configuration.
Otherwise, as I missing something?
The only thing I feel I'm missing over over a real server would be an IPMI so I can boot it remotely in the event of a power loss. There are solutions to use a rPi for that which I plan to look into... some day.
Let's be honest, it's frighteningly easy for a rack to turn into a giant ball of wires too. See r/cablecore for examples.