I had some questions about the "Roman Bread" on my Pompeii food post, including what the string that bound the loaves was for and had I ever tried it. I don't tend to like working with wet/slack doughs or sourdough, both of which the Roman bread was, but at the time I did have two loaves of beer bread rising, so I figured I'd try out the string technique. This beer bread is a relatively soft dough that normally bakes inside a pre-heated cast-iron pan in a very hot oven, which is actually similar to how Roman bread would have baked.
[ID: Three photos of a loaf of beer bread; in the first, the unbaked dough is sitting on parchment, bound with a string around the outside and with scoremarks in the top to divide it into wedges. In the second, the baked loaf is resting on a cooling rack, the scoremarks evident but not overly deep; in the third, I am holding the bread by the string around its edge, as it dangles sideways in the air.]
The most widely-held theory is that the string allowed a customer to carry the loaf easily, although bakers have pointed out (and I believe classicists generally agree) that binding the dough also gives it more structure. The loaf definitely stood much taller and baked up "higher" with a binding made from doubled-over butcher's twine than it would have normally. I scored the top with a lame, but the scoring didn't hold as true as I'd have liked; bakers who have done more work with Roman bread generally don't score it this way and instead use a floured dowel to press all the way through the dough and then let it come back together, which is how they believe ancient bakers did it.
Some bakers have said that tying the string around the middle also allows you to rip the loaf lengthwise, splitting top and bottom; mine didn't work super well for that, but I also was working with a stronger crumb, I suspect.
But yeah, all in all, the string is useful if you want to give your loaf more structure and held the bread well enough that I certainly could have carried it around an ancient market if I'd had a mind to.
