Why are there holes in Italian buildings and towers?

When I worked as a tour guide in Italy, I got this question pretty frequently from travelers who paid attention to architecture.

Why are there holes in Italian buildings and towers?

The photo below is of the clock tower in Verona. If you look closely, you’ll see that the tower is full of little black squares. They’re sort of holes, but they don’t go all the way through. They’re in most towers, and in the unfinished parts of many churches.

Do you know why they are there? Post in the comments, and I’ll update this post in a couple days with the answer if it hasn’t been posted.

Here’s another photo where you can see the holes, in the Basilica of San Petronio in Bologna.

“San Petronio” by Massimiliano Calamelli via Flickr, licensed under CC BY-SA 2.0

Update, October 17th:

Thanks so much for all the thoughtful and imaginative comments! Let’s take the towers first….
Dominique is correct in saying that the holes were slots for some sort of insert. In fact the holes were used for the scaffolding during construction. When the tower was completed, the scaffolding was removed, leaving behind the holes. Medieval towers were built as lookout posts, and their height reflected the power of the family or clan who built them (who would also want to reattach scaffolding and build the tower higher whenever they could afford it).  Most were not designed to be decorated with prettier stone, unlike church facades.
Here’s another photo, of two of the many towers in Bologna.

As for the church façades, I had to go do a little research!

I had always assumed that the holes in churches and the holes in towers were both due to scaffolding, but after reading the comments below, it seemed logical that holes may have been for attaching the façade to a church. Rene is correct that most brick church façades were intended to be covered with prettier stones at some point. And it is true that marble and iron (and more) were pillaged from buildings. But after some internet research, I found nothing about the method of attaching church façades, and nothing about the reason behind the holes or indentations in unfinished church façades.
So I can’t say for certain what the holes in unfinished church façades were for. Since they are the same size, shape, and have more or less the same spacing as those in the towers, it makes sense they would have been for scaffolding too. (And on the unfinished façade of San Mercuola in Venice, the holes start at about 8 feet high, a logical place for scaffolding to start.) But I am now very curious, so please post if you find a good link on this!

Photo of Verona clock tower by Sanjay Jhawar, photo of San Petronio by Il Cama (crop by me); photo of Bologna towers by Sanjay