When drinks are made on the rail, bartenders can keep their hands and mouths away from the glass which means that any diseases they might have won't be passed to the drinker.
This is best for protecting consumers and barback handling beverages by putting it in a way so their mouths never touch the beverage. There's also potential problems with contamination from food residue if you're making mixed drinks, as well as exposure to highly polluted air if your restaurant or bar is near an industrial area frequented by vehicles' exhausts. Last but not least, there's less strain on your bartender's wrists because they don't need to do much heavy lifting!