Ceres is the only known dwarf planet in the inner solar system, and may have liquid water, which is possibly why it was chosen as a planet with missions.
Missions on Ceres are named after different Roman/Greek-relations and numerous asteroids.
Ceres is the highest-leveled Grineer-controlled planet in game.
Ceres is a very interesting and strategic point for the Grineer to build their ship building operation. Due to the dwarf planet being located in the Asteroid Belt, the Grineer can easily mine the bounty of Mineral and Metal-rich Asteroids to fuel their operations. The small planet also has plenty of ice, and the Grineer have warmed the temperature to allow water to flow.