Dextrose Monohydrate is a monosaccharide, a simple sugar, and is used as a building block for biological structures or can be broken down to power life-sustaining biochemical reactions.
During the production of beer, the mashing of grain breaks down many compounds with starch comprising a bulk of the targeted compounds. The starches are broken down by enzymes into constituent parts, and some of these are dextrose molecules.
During the kettle, some dextrose binds with nitrogen-containing substances in a colour- and flavour-forming Maillard reaction then consumed by yeast during fermentation and in turn, yeast releases alcohol, carbon dioxide, and flavour and aroma active compounds.
Why would we want to use anything other than the sugars that come naturally from the barley? Well there are a few reasons that may apply in some circumstances: a.) To raise the alcohol level without increasing the body of the beer. b.) To lighten the body of the beer while maintaining the alcohol level. c.) To add some interesting flavours. d.) To prime the beer for carbonation.