Even if you're just writing standard native apps with Xcode, being able to build, test, and deploy your app every time you push to the production branch in your Gitlab/Github repo is valuable. With codesign on Linux, you can build that on top of a simple, ~5mb alpine linux container.
Common practice for CI is to actually run tests, though. How would you validate your macOS/iOS builds are functional without still requiring VMs or real hardware?
Then after merging, you have a second machine / set of machines that only does signing. It increases the difficulty of an accidental or intentional leakage of your sensitive keys.
(Note: I have not tested this yet)
Being able to sign on Linux will allow us to re-use the existing signing infrastructure we use for literally every other platform other than MacOS. It'll be more secure and much less maintenance.