Facebook rolls out UI changes in less valuable markets to gather data to verify their changes don't decrease engagement.
In the 19th- and mid-20th centuries, all of the big advertising agencies handling consumer goods were headquartered in Chicago (many still are).
They would test products by introducing them a couple of hundred miles away in Peoria, Illinois. If the items were well accepted there, then they were rolled out to the rest of the country. It's where the expression, "But will it play in Peoria?" comes from.
This still happens. Back when Starbucks was still cool, it would roll out new drinks in Chicago and a couple of other cities as test markets, and then later in the rest of the nation if they were successful.
And props for history less on "But will it play in Peoria?"