Slipstream Runners are very large, smooth, rails that are used to help ride the quantum strings which make up the Slipstream. They are not completely essential for using slipstream as FTL travel, but they do make precision steering easier and make the ride a whole lot smoother. Smaller ships, such as Drones, Slipfighters, and cargo vessels like the Eureka Maru cannot be designed with slipstream runners because the ship needs to be a certain size in order to be effective. Extremely large vessels, such as the Magog Satellite Ship and Magog World Ship, have no need for runners to provide stability and speed, as their colossal mass of trillions of tons anchor them in the slipstream, and make them extremely stable.
The use of slipstream runners is completely exclusive to Systems Commonwealth vessels, and only then on the largest ships such as ships-of-the-line and enormous cruisers. Slipstream runners are usually an integral part of any ship that they are built into, and they give the ship a very sleek, swept back, and streamlined design. The Commonwealth used slipstream runners because, while being technologically useful, the sleekness of the overall look appealed to the designers' vanity.
The master Vedran shipbuilders, and the massive orbital shipyards took pride in designing the most beautiful, as well as deadly spaceships every created. Slipstream runners are very sleek and smooth, and also help to facilitate the art of piloting slipstream. So, slipstream runners were put into the designs of the largest Systems Commonwealth vessels in order to make them faster and better than any of their counterparts.
Note that all of the vessels are very large ships - some of the largest vessels in the Commonwealth's fleet, and even the known universe. They are also very powerful, and are often used as shows of force to help bring weight to any negotiations or disputes that the Systems Commonwealth is trying to mediate or is participating in.