I built a four-node Raspberry Pi cluster this weekend. It runs k3s, it has a little OLED screen that shows CPU load, and it taught me absolutely nothing I will ever apply to anything. I had a wonderful time.
The honest reason these things exist is not learning. You can learn Kubernetes far more cheaply with three VMs on a laptop, no soldering, no PoE HAT that arrived with the wrong pin header, no afternoon spent discovering that two of my four SD cards were lying about their capacity. The cluster is a physical object that does a computery thing, and it sits on the shelf blinking, and that is the entire payoff. I am at peace with that.
What I will grudgingly admit it taught me: cheap SD cards are a menace, and you should burn the same image to all of them and checksum the result before you trust any of it. One card passed dd and then quietly corrupted itself under write load three hours later. The little OLED was the genuinely fun part: an I2C display, a dozen lines of Python, and now I can see four ARM cores being heated up by a workload that does nothing useful. Marvellous.
Would I recommend it as a way to understand distributed systems? No. Would I do it again? The moment I see another PoE HAT on offer, yes.