Yesterday, Microsoft released Windows 10 Insider Preview build 18965 to the Fast ring, not including much in the way of new features. Just a day later though, a cumulative update for the build is rolling out. KB4517787 brings the build number to 18965.1005.
As noted in the blog post, this cumulative update doesn't include anything that's new, although Microsoft did say that the size of the update will be a little bit larger than a standard cumulative update. According to Brandon LeBlanc, who penned the blog post, the sole purpose of the update is to test out the servicing pipeline for big updates.
This is nothing new. With almost every major release of Windows 10, there are a few Fast ring builds that get cumulative updates. After all, servicing is one of those key things that needs to be tested, since it has to work properly when the update is available for the general public.
Windows 10 build 18965 really only had one new feature to begin with, which is that users have more control over apps restarting at sign-in. There's a revamped Feedback Hub as well with some UI changes, but that update is rolling out more broadly.