It won’t surprise anyone that toast has been around for as long as there’s been bread: about 6,000 years. The Pharaohs ate toast, so did the Greek philosophers and the Emperors of Rome. The Merovingian Kings, the Holy Roman Emperors, Renaissance painters, New World explorers, Napoleonic warriors and so on. Indeed one could argue that throughout all the ups and downs of Western history, toast has been one of the few constants. No wonder we like to wake up to it in the morning.
For most of history toast was little more than a strategy for extending the life of bread. Exposing it to heat removes its moisture making it less susceptible to molds. Plus heat makes bread rigid, less likely to crumble in the pocket.
That said, toasting does a lot more that remove moisture. As bread browns its flavor changes from bland and starchy to wheaty and nutty. Just how this happens has been a subject of intense debate over the years. Some claim it’s all due to caramelization (the molecular breakdown of sugars) while others maintain that the Maillard reaction (the breakdown of proteins) is responsible. Either seems plausible to me since bread is on the one hand full of complex carbohydrates (sugars), yet it also contains about 10% protein (gluten). My guess is both are probably happening right this second in your Black & Decker.