It's a totally separate domain. Pharo/Smalltalk is essentially pure OOP in the way that Scheme is pure FP/LISP. So for me personally, I've used to increase the depth of my understanding of programming. It can also be good for prototyping class hierarchies and the like.
You can find examples of people using Pharo in production (and far more examples of people using other, more industrial-grade Smalltalk dialects), but I personally haven't been able to think of a way to use it on real projects. But that doesn't mean that it isn't worth studying.
One reason would be for trying out the environment, which represents a totally different approach to programming than the traditional file based and textual approach, instead of having your coding and runtime as part of the same environment, where you're communicating with live objects.
You can find examples of people using Pharo in production (and far more examples of people using other, more industrial-grade Smalltalk dialects), but I personally haven't been able to think of a way to use it on real projects. But that doesn't mean that it isn't worth studying.