B'reishis Rabba (90:6) indicates that Yosef's decree was designed to provide the Egyptians not only with life in this world, via physical sustenance, but with life in the World to Come which they could merit via circumcision.
The Y'fei To'ar commentary (ad loc. and on 91:5) explains this by saying that the Egyptians were steeped in sexual immorality, of which their foreskins were emblematic. By removing the foreskin, the Egyptians lost their symbol of promiscuity. Yosef intended that circumcision would thus temper the promiscuity within Egyptian society.
Another suggestion of the Y'fei To'ar is that Yosef wanted to make it easier for exceptional individuals who would later decide to convert and join the Jewish people. Because the ordeal of a circumcision can be a difficult hurdle for a prospective convert to surmount, Yosef forced all Egyptians to be circumcised to remove an obstacle from the path of individuals who would want to convert.
The Maharazu (91:5) provides an alternative explanation for Yosef's decree. He says that divine blessing and salvation from famine came to Egypt and the surrounding countries via Yosef's extraordinary merit, which derived from his repulsing the advances of Potiphar's wife. This merit was symbolized by the covenant and circumcision of Avraham. Thus, it would be incongruous for Egypt to enjoy this merit without likewise undergoing the process of circumcision.