I bought a Turing Pi 1 . It's a 7 node mini-cluster, not very expensive (I think about £300 fully populated), that fits into an MiniITX-style case with a single small power supply and single ethernet exposed to the world. You can easily pick the thing up with one hand. It's certainly not powerful, each node is a very modest ARM processor with "only" a gigabyte of RAM and 8 GB of local storage.

The alternatives are much more powerful, but far more expensive and not very attractive in other ways. Blade server? Huge, noisy, power-hungry and I wouldn't get much change from many thousands of pounds. Rack of NUCs? Powerful of course but far more expensive, large, separate power bricks, ethernet switch and lots of wiring required. VMs running on a desktop? Still more expensive, larger, and not the same tangible experience as having a real cluster. VMs on a laptop? Laptops at the same price point would have too little RAM to run lots of VMs well.

So these Raspberry Pi clusters fit a niche which is not well served by other solutions.