Originating as a joint venture by turian and batarian interests in the Terminus Systems some forty years prior to the Reaper War, the Ea-Dagan/Cartha Rig is the culmination of literally decades upon decades of experience in the field of mining. Hardy and adaptable, possessing industrial grade heavy lasers, hardened kinetic barriers to ward off the clouds of debris, detritus, and other natural hazards that surround mining sites, mass effect tethers to anchor and/or suspend chunks of asteroids and planetoids, drone bays, and basic processors, the Rig is optimized for harvesting large quantities of raw ore and transporting it to secondary refining sites (usually a nearby mining or constructor platform).
There are currently a great many variants. The first significant deviation from the standard design was the realization that, if the onboard processors were swapped out with fabricators, the Rig could be repurposed into an effective engineering and manufacturing ship.
The second was the equally helpful realization that integrating a basic accelerator track onto the Rig's superstructure along with a handful of redundant heatsinks put a solid, reliable, defender at your disposal that could give any flotilla of normally vulnerable craft both a shield and teeth.
Take into account the regular hardware updates and newer, more efficient models released every couple years and it's easy to see how the Rig has maintained an enduring sense of popularity both in C-Space and the Terminus for the better part of a century.