They were constructed during the closing stages of the War in Heaven with their design orchestrated by the C'tan known as Nyadra'zatha. The Burning One had long desired to take his eldtrich fires into the Webway itself and is known to have taught the Necrons how to breach its walls. A series of such devices were constructed which allowed the Necrons to turn the Old Ones' greatest weapon against them, which greatly accelerated their defeat.
Their nature means that they are both unstable and uncontrollable when compared to natural entry points into the Webway. In an unknown way, the Webway even detects breaches into itself by the Dolmen Gates and moves to seal off the infected spur until the danger posed by it passes. Thus, any Necron entry into the Webway must move quickly to reach its destination, otherwise the network itself will bring about their destruction. In the aeons that have passed since the War in Heaven, the Dolmen Gates became lost or abandoned during the Great Sleep or destroyed by the Eldar whilst the Webway itself has become a tangled, broken labyrinth.
The remaining Dolmen Gates only grant access to a small portion of the Webway, with much of the network being sealed by the Eldar to prevent further contamination. Despite this being the case, the immeasurable length of the sundered Webway does allow the Necrons to outpace the modes of travel used by the younger races. As they are bereft of Psykers, the Necrons rely on this method of transportation as they are incapable of Warp travel. Thus, if they were denied the use of the Webway, the Necrons would be forced to make use of slow moving stasis-ships that would doom their civilization to isolation.