Why are there holes in Italian buildings and towers?

The photo below is of the Basilica of San Petronio in Bologna. If you look closely, you’ll see it 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.

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.

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. But 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!