It's all part of making them addictive. It's similar to why something as simple as "apple juice" contains more than two ingredients. Any quick look at the side of most apple juices will show a list of barely recognizable compounds, chemicals, and other preservatives. Reason being, they need all that to maintain the "goodness" of it. Goodness being the bare drinkability of it, basically in that if they don't add over 9000 chemicals, it will be even worse than it already is.
In the same vein, cigarette companies add in dozens of chemicals and other crazy stuff in order to maximize the enjoyable, addictive quality of their cigarettes, in the most efficient way possible, hence using things that are poisonous or downright "what the hell?". I don't know the exact specifications of that garbage, such as why tar or urea is found in them, but it's all a big corporate grandma's recipe, just like you'd find in other markets like fast food. (I.e. why just one mcdonald's hamburger can contain meat from nearly a thousand different cows)