In the industry anything taller than you are is "vertical real estate" and loaded with antennas of every shape, size and description. If you have to bend down to see the roof likely they're cell panels that pop up everywhere to fill in holes and give adequate coverage. It's called "site leasing" whereas the owner of the antenna(s) and equipment rents space from the landlord. I know a couple of hams that lease tower space to commercial outfits and the majority of ham repeaters are at leased sites.
An interesting thing to note is cell sites and the coverage of each cell varies widely. In rural areas you'll see very tall towers miles apart so each cell covers a wide area while in urban areas and inner cities in particular you'll see antennas everywhere so each cell may cover only a few blocks, sometimes even less. Since they hand you off seamlessly you never know when you move between them, it appears as continuous coverage to the end user. Why so many in urban areas? Penetration much the same as with those systems you monitor, they want coverage deep inside buildings which is why PS systems in these areas also have so many "tower" sites.
