Ok, I suppose some may respond with the answer that these are there to keep the engine balanced and running smoothly. I just saw a picture of the 2.0L engine apart and see that there are two chains in the engine. One from the crank that goes to the two camshafts. There is another chain also driven from the crank that goes to two "balance shafts" and there is mention that they are to help balance the intake and exhaust. If so it seemed odd they are located where they are and that they are driven by a separate chain. Is there some other purpose for these? I've never seen anything like it.

The 2.0L seems a lot more complex than I first thought. I'm not too worried at the moment since I have heard they are study little engines but I would like to understand why all the pieces are there.

They are there to reduce noise & vibration and the chain also runs the water pump.

In my Sentra, there were two balance shafts that rotated at 2x engine RPM to quell the vibration and harshness of a large displacement (2.5L) I-4. A popular mod was to cut the drive chain (worth 8whp) and possibly remove them for weight savings. They also caused problems because they foamed up the oil at sustained high rpm, such as in road racing. Jim Wolf Technology, a very venerable Nissan tuner, offers a kit to remove them and maintain proper conditions in the oil pan. I would not be surprised to see something similar for our cars sometime in the future.