TL;DR;
This is needed because of the issue with rendering expect/actual. I would propose for now to just add macosArm64 target and resolve underlying issue during implementation of #3386 for Dokka K2. We will anyway need to do something with such sourceSets.

In #3662 test started to fail on ui-showcase module because of ERROR CLASS.
Checking the page where the fail happened, I found that for some reason ui-showcase currently (on master) doesn't contain an actual declaration for macos sourceSet:

asyncWithDelay doesn't have macos tab, but actual declaration exists in sourceSet.

If we run the same test with the changes from #3662, macos tab is now appeared but shows ERROR CLASS:

All other declarations of asyncWithDelay for other sourceSets look the same as before.

Also, other declarations (which don't depend on coroutines) like getCurrentDate which also has expect/actual defined in the same - work fine on master and macos tab exist:

Most probably the issue is very similar to #3386 (even though problem described here reproducible on K1).

If we build ui-showcase with Dokka K2:

• on master - linux and macos are not shown at all for asyncWithDelay.
• with Correctly handle case when we have internal expect but public actual declarations #3662 - all source sets are shown but both linux and macos will show ERROR CLASS
• if we add macosArm64 target (as in this PR) - macos source set will be shown.