I noticed four(?) towers around the SpaceX launch pad during yesterday's aborted launch
Image credit NBC News
What are the towers used for? Tracking? Radio communications?
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They are used to redirect lightning in the immediate area. This essentially creates a faraday cage, shielding the rocket from being fried by lightning. You can see how high the towers reach, high enough to ensure there is no risk of lightning hitting the craft.
Update by @highonrope: The rectangle which the rocket launches through is huge...from the ground to the bottom of the "candle stick" is 250 feet tall. The candle stick is the big white thing at the top. It is about another 150 feet tall. They look a lot smaller in a picture, but it takes several minutes for an experienced tower climber to go from ground to the top. They say that inside the lightning suppression system there, you are 100% safe from lightning. The towers are all interconnected with stainless steel cables which form a rectangle through which the rockets are launched from the rectangle, the cables run to the ground and are fully grounded. @highonrope was part of a team of people who were tasked with cleaning and repainting the top of the tower and testing the "ground" of each cable where it attached at the top and at the ground, when SpaceX first started at Kennedy, before the first launch there. We got to see first hand how huge the SpaceX rocket was and what it does to the pavement in the "exhaust" tube, where the rocket exhaust is redirected at ground level. It's an awesome complex. It was designed for the Saturn rockets and is one of the oldest launch pads at Kennedy.
TL;DR: They're lightning rods.
The big top rods are insulators. Such cages are grounded through wires sloping out to ground points several tens of meters away. Also the ground points are connected to each other by a potential equalization loop. The metal lattices of the towers are grounded too, and also connected to the loop - but they are insulated from the tips, so in a strike they do not act as conductors. There was one such insulator on the top of the Shuttle launch tower. It was a spun fiber in polymer matrix structure, ad the time the biggest such cylinder in the world. The tip was connected to three ground points.
The first piece of Pad 39B to be removed, the 80-foot lightning mast that topped the fixed service structure (FSS), was lowered on March 3, 2009. NASA's first launch pad lightning protection mast, it was used for the 1975 Apollo-Soyuz Test Project (ASTP) before being adapted for the space shuttle program.
Construction of three lightning towers continues at Kennedy Space Center's Launch Pad 39B in January 2009
The fiberglass (FRP) tubes on the top of the launch platforms are a part of a lightning protection system and are insulators with cables that form a cone of protection for the launch tower. They were designed by my former company CH2M HILL, (Gainesville,FL)an engineering firm, and were manufactured by Starline Fabricators, Astatula, FL. I brought this design project in for the company from a contact I made with the fabricator during a business development trip to Lake County in the late 70s.
This is a complement to the already given answers, namely that these are supports for a lightning protection system.
Below is a picture of an ULA launch that shows that besides the spires themselves, there are wires stretched over the launch pad to redirect any lightning that would want to strike the rocket. It makes the role of the towers much clearer.