Back in the days before the Windows Insider Program, one of the problems that frustrated the product team was build leaks. These were builds not intended for public consumption, and their existence was pretty much all downside. The first impression the outside world got of a new feature was the buggy version, which was also probably quite ugly on top of being buggy. That is rarely a good first impression. The feature arrived without context; people often jumped to conclusions about what the intended purpose was. The team didn't get a chance to talk to partners in order to give them a chance to raise their concerns about the new feature. The premature disclosure meant that the team's big announcement event no longer had the impact the team wanted. And there are some legal issues that are tied to the date a feature first becomes available to the public. Seeing a feature go public prematurely throws a bunch of scheduling into disarray because you now have to finish those legal documents in less time than you planned.
A member of another team told me that there was a leaked build that had a really bad bug in it. I forget exactly what the problem was, but the details aren't important. The team fixed the bug as soon as it was discovered, and they notified all the self-hosters and partners to upgrade to a new build immediately, but that bug was also out there in the leaked builds, ready to destroy computers and networks and most of Western civilization.
So how do you get people who are running a leaked build to stop running that build, with urgency?
Change the wallpaper.
The feature team asked the shell team to change the default wallpaper for the build that has the fix.
This is sort of the converse of If you change the insides, nobody notices: Changing the outsides counts as a major change.
The theory was that when the new build comes out with the new wallpaper, whoever it was that was leaking builds will say, "Whoa, this is a big deal," and make a special effort to leak the new build, and then the underground sites who traffic in leaked builds will see that the new build has a new wallpaper and say, "Whoa, this is a big deal," and they will abandon their old and busted build with the horrible bug and install the new hotness.