If you are interested in distributed systems/programming problems they are fun for that. I have setup 5-6 Pi's before and written different leader election methods and play with pulling their network cables etc. I also wrote a partially custom service discovery using etcd and tested it on them, again mostly for learning but it led to some production code later on. I've played with data consistency problems using them too. Also worked good for testing system recovery when I was writing code that did graceful recovery from failures, lot less at risk if you pull power from a Pi then a server to test the code. :)
Outside of that, you could connect them to some relays and do some electrical switching via the network etc. Or setup a few of them with the camera and do image classifications or just motion detection.
