They are essentially for kids to play around with learning computers by blinking LEDs and integrating with circuit boards. The idea of building a high performance cluster with pis is dumb from day one
Where a "kid" may be a 53 years old with 30+ years softdev experience who ultimately got to get to the stuff he wanted to for quite some time, and the "blinking LEDs" are a bunch of servos programmatically controlled based on input from a bunch of sensors. While there are definitely better alternatives based on various narrow metrics, especially when it may come to actual productization, the ease (and cheapness, so you don't think much about that spending) of starting with all those easily available for RPi servo array drive boards and various IO ports array boards and all the available software - it is hard to imagine how it can be more easy/cheaper/available than it already is with all that actual compute power and full-featured Linux environment.