Today, Microsoft is releasing a new cumulative update for Windows 10 Insider Preview build 21327, bringing the build number to 21327.1010. Unsurprisingly, the announcement says that nothing is new, and that this is just to test out the servicing pipeline. Of course, there have been several instances where the company has said that, and there have been changes that Insiders weren't told about.
It does have to test out the servicing pipeline from time to time though. It just need to make sure that it's still possible to service builds with cumulative updates, a fundamental part of how Windows 10 works. And if the company goes too long without making sure that it works, and it ends up being broken, it's a big problem.
This update is KB5001277, and frankly, there's nothing else to say about it. This is all in the Dev channel, meaning that it's not tied to any feature release of Windows 10. It's all in a perpetual state of being from the rs_prerelease branch, which means that those Insiders get to try new features first.