Toasting provides additional flavor and exterior crispness that can enhance the sandwich. But the real reason to take this step? By the time you pull one out of the bag, most burger buns have staled at least slightly. Toasting returns the interior to its fresh-out-of-the-oven softness, creating a supremely plush, moist cushion for the meat.
Why? Heating bread to at least 140 degrees temporarily reverses retrogradation, the process that stales bread and other starch-based foods, by releasing water that gradually gets trapped within the crumb’s starch crystals as the bread sits. Once it’s released, that water is then available to hydrate the crumb, making it taste more moist and fresh.