Apart from the Xcode toolchain, I cannot for the life of me think of a single macOS application that both has no GUI _and_ isn't already cross-platform or a Darwin/BSD version of a tool that already exists on Linux.
I'd actually like to know what non-Xcode command line apps you (or anyone else reading this!) is using that explicitly target macOS and nothing else - I've used Macs for only about five years and primarily for dev work, so maybe there's a treasure trove of cool stuff I'm missing.
I mostly had the same thought as you, but one thing I imagine some people might want is access to Apple's own command line tools, particularly those who prefer using Linux but need to work with Apple formats for some reason.
There's osascript for executing Applescript code, plutil for working with plists, and diskutil for making DMGs. Those are just off the top of my head, there's probably others too.
Hmmm - that's all fair! Though I'm not very sure why one would want to execute Applescript on Linux - scripts are typically either so platform-integrated that they're useless on non-Mac platforms or so generic that they're trivially replicated in languages like Python or Ruby.
I'd consider diskutil and plutil vaguely part of the "Xcode toolchain" (as in, using them (especially from outside macOS) is largely geared towards app development). That's certainly a huge use case for Darling - I actually think it would be interesting if they explicitly (but maybe not very loudly?) aimed for giving people access to iOS/macOS development without Mac hardware, rather than trying to be a general-purpose emulation layer.
I've been using OS X since about 2002, if I'm remembering right -- I bought my first Mac in 2001, I'm pretty sure (one of the crazy "clamshell" iBooks with a PowerPC G3), and OS X was not really suitable as a daily driver until 10.2 "Jaguar" in August 2002.
I mention all this because even with that nearly 18 years of experience, I am really straining to think of a single non-GUI application for macOS that wasn't a Unix command line port. There are some Apple-provided system utility commands, and, fine, but probably of pretty marginal use in this context.
I think it's a great project, don't get me wrong, and I wish I had the system-programming chops to help. But it still seems to mostly be a proof of concept. I hope it keeps growing!
I'd actually like to know what non-Xcode command line apps you (or anyone else reading this!) is using that explicitly target macOS and nothing else - I've used Macs for only about five years and primarily for dev work, so maybe there's a treasure trove of cool stuff I'm missing.