Because the Earth gets hotter the deeper you drill, EGS could expand the reach of geothermal power enormously and provide access to a virtually inexhaustible energy resource.
With each new well, new tools are being developed to deal with the intense pressures and temperatures involved with controlling a drill bit that is 6 miles into the earth.
ExoMars, as we call it on this side of the pond, envisages a satellite to study the Martian atmosphere, followed by a large robotic rover to drill beneath the surface and cache rocks for later return to Earth.
That was as deep as one could look into the earth with the available geological tools--and also about as far into the ground as anyone knew how to drill a well and push pipe into the hole.