The point to purchase city slabs is so you build facilities, to get the income. If you own all the buildings in the city slab, you have to have one building that produces income. So not from the city, but the facilities within.
Many of the cities on the NPC market are only about 17% filled, .. lots of space to build. Also, buying a city, putting flats on it, if you are a faction, you then take over the planet, and no one could build there except you. Another perk if you own a faction and want 'cheap' control.