The baking soda causes an explosion of gas in the hot syrup. If you can get it out of the pan and onto a greased sheet pan fast enough, you get a lighter product. Syrup alone cooked that hot will be like glass, clear and brittle. Depending on how much baking soda, you can get something almost like volcanic foam.
Too much soda and you can really taste it though. I have a recipe for some stuff from Australia called honeycomb which really takes it to the limit, if you're interested. Quick and fun to make.