[Thingol]

I believe that Saruman's ring allowed him to better control his armies and servants. Saruman was attempting to imitate Sauron and it is likely that the ring he made was like the one ring, but of a lesser power. The one ring allowed Sauron to dominate the wills of his servants and to dominate the wills of all of the other ring bearers. Saruman's ring was certainly not powerful enough to allow him to dominate the wills of the ring bearers, but it probably enhanced the power of his voice, allowing him to control his armies. The rings of power gave power according to the stature of the bearer and it makes sense that Saruman's ring would increase the power of his voice. After all, keeping an army of orcs under control is probably much harder than the bad guys make it look.