It's regional varieties -- just as there's different cheeses and olives, different types of pastas have evolved.
Now, you might think at first that it doesn't make sense, as we could produce the same pasta everywhere, you have to remember that Italians will pair specific sauces with specific pasta to get a specific balance of sauce to pasta. As different things are grown / raised / etc in different areas, we end up with different pastas preferred for different sauces. Some are better for oil (eg, thin strands, light and delicate), some for cream sauces (wider strands, tubes), some for tomato sauces (wider strands, anything with good ridges on it), some for soups (smaller shapes), etc.
Now, I know, there's lots of tubes, and there's lots of strands ... but others could ask why Americans need Coke, Pepsi and RC Cola (although, in that case, there's commercial interest, but there's still some heavy regional preferences out there, particularly for smaller local varieties)
As for the difference between penne vs. ziti -- the angle of the cut on the end can affect how much sauce gets inside the pasta, as the angled cut of the penne will grab some as you're stirring the pasta in the sauce. (or so I've been told ... I've never done a side-by-side comparison of two with the same diameter / thickness / length, etc.)