The Cadian Pylons were ancient warp-containing devices of Necron origin; the pylons had the power to contain warp phenomena and even shut it out completely, protecting the necrons from warp-based powers, which is the only sort of capability they lacked completely.
While the name suggests the only place you could find the Pylons is Cadia, this was not the case; they were actually presented in many different locations across the galaxy, apparently creating a sort of defensive envelop around the Eye of Terror. If you consider the Eye was originally Eldar-held territory, it makes complete sense in the context of the War of Heaven why the Pylons were placed in their respective locations, or to be more precise they actually formed a sort of containment frontline defense for the necrons during the later stages of the War in Heaven, with the ones in Cadia working as the central node of an interstellar grid.
The Fall of Cadia reveals Abaddon's long-term plan was to slowly destroy the Pylon's grid, breaking the pillars of creation, allowing not just the Eye of Terror to expand but for the material universe to lose its resistance against the Warp, which explains the increase of mutation, sedition, and Warp Storms during the ten millennia of the Imperium's history.
What is more, the Pylons may have been the reason why Warp phenomena was not recurrent during the earlier ages of mankind and the Pylons are one of the main reasons why the decadence of the Eldar didn't completely engulf the Galaxy into the Realm of Chaos; unfortunately, with their destruction, this may not be the case anymore. Fortunately, the rise of the warp will likely encourage the Necrons to do something about it. So, everyone else can sit back and watch the Necrons fight Chaos, watch the Tyranids join in, and the Orks come to the big fight, then come in and kill off anyone who survives.
The Fall of Cadia also indicates that the pylons can be used to close the Eye of Terror.