(Ellen e-mails Mr. Answer Person: "This is on Upper Broadway, near Mrs. Basler's house. I haven't heard any announcement from thePost Office. Did I miss it?")
Mr. Answer Person says, "Actually, decorahnews.com posted a news story about these centralized mailboxes one year ago (http://www.decorahnews.com/archived-stories/2014/06/8341.html), but it's not like you're required to take a test after reading our news!
At that time, the centralized mailboxes were put out in business areas. But as we noted at the time, "The multiple mailbox units will become more common in Decorah in the future."
In talking with Decorah Postmaster Lee Moser on Monday, Mr. Answer Person has learned that last year's prediction was accurate. The U.S. Postal Service plans to add more such mailboxes because they enhance the safety and security of mail carriers (no dogs to contend with!) and help to lessen the odds of slips and falls by mail carriers.
But Moser says there are advantages to homeowners, too. The Postal Service is aiming to eventually install centralized box units in all of the downtown area because carriers don't have to wait until a business is open in order to deliver mail. That means businesses can get their mail before they open their doors for the day.
As you note, the centralized box units have now begun to appear in residential areas, as well. However, the boxes are being installed first near new home construction. Moser also says the Postal Service is sending out letters to customer input before any box is installed.
So, to directly answer your question, the centralized mailbox on Upper Broadway was installed because of new construction in the area--and only after neighbors were consulted.