Do breadcrumbs really act as a binder in meat preparations? I routinely hear that breadcrumbs are added to burgers and meatballs to bind them.  For example, see the answers to this question: how to stop meatballs falling apart. On the other hand, I have heard professional chefs say that the bread does not bind the meat. The egg is added to bind, the bread is added as filler.

I have never put bread or breadcrumbs in my meatballs or burgers and I don't have problems with them falling apart. I do however, put an egg yolk in and work them long enough that they don't break when pressure is applied.

So which is it? Filler, binder or both? If it's a binder, by what mechanism does it bind the meat?
Actually it is neither filler nor binder, but moisture retainer. Breadcrumbs in your ground meat will absorb much of the natural juices during cooking and then will release that moisture back into the meat as it rests resulting in a 'juicier' burger, meatball or meatloaf.