A standard engine already prepared, such as the Quake 3 one, wasn't good enough for Westwood and their first foray into the 3d universe with the universally known Command & Conquer series. They could have taken a quick way out, with little expense, but they bravely chose to start their own engine, from scratch, so that every detail was perfectly suited for the intricacies of C&C Generals. So far, it seems to have paid off.
The benefits of the "Sage" engine against other widely used engines are its flexibility. It can push the polygons just like any other, forming beautiful explosions, renders and shadow effects, but what really shines through is how it is considerable for those on lower end graphics cards that don't have the raw power of the latest Geforce 4's. The "Defcon" system lowers the special effects when units are crowding the screen, and all hell has broken loose with the formerly restrained GLA soldiers. The theory is that players don't care about a real time shadow when their precious Crusaders are getting owned, they just want to finish the fight. It certainly makes sense.
It is also rumoured that the Sage engine will be used for the next Command & Conquer RTS - one rumoured to be set in the Tiberian Universe and possibly already in development at Westwood, Las Vegas. However, nothing has been confirmed as of yet - it is even possible that this project was started and then cancelled. Nevertheless, the engine should prove a big hit.