bread is a gel! when it gets stale, its because the water evaporates and causes the starch to crystallize! heating the bread melts the crystals, and adding some water helps re-gelling as well.

Putting in a little bit of water helps with reheating a ton of leftovers, it can make anything more moist that has been dried out. I like doing it with pizza and chicken wings a lot.

You can also freeze bread or rolls and do it after you thaw it out.

I usually just cut it into chunks, toss with oil and spices and make croutons.