Right now people mainly use it for web apps, sometimes data processing. With windows support, I think there would be a rush to make cross platform GUI apps.
I love crystal and I have a lot of autonomy so I get to use what I want. Shainet is also really good in general for coming up with new network topologies, which I find is a lot harder to do in a lot of the python-based mega-frameworks, Cafe, etc. In general, fibers are really good for concurrent/parallel data processing, and now that crystal has true parallelism (enabled with a flag), for my purposes there is no reason not to use it.
Occasionally I will use rust or C/C++ for some of these tasks, but I try to keep things in crystal whenever I can.
EDIT: Took a little digging but I found it [1] (and yes it's primarily x86_64 linux+macOS)
[1] https://github.com/crystal-lang/crystal/wiki/Platform-Suppor...