Why does the Vatican city have an obelisk in it?

The Vatican is a country unto itself, and full of all the same political idiosyncrasies of any other nation. Pope Sixtus V had the obelisk moved to its present location in the 1500s. I'm not sure why, but like much of the statuary in and around the city, it is pagan in origin. It also dates back to the time of Augustus, and was a symbol of Roman might. To this end, it probably became symbolic of the papal authority when Sixtus moved it to the center of his world.