Why did greek scientists study the circumference of the earth

Greek philosophers believed that the Earth was a sphere because they believed that the sphere was the perfect shape.  Eratosthenes was the first to calculate the circumference of the Earth because he was interested in geography and wanted to create a map of the world.  In order to create the map, he needed to know the circumference of the Earth.  He used the angle of the sun during the summer solstice and then the measurement from Alexandria to Syene to calculate the circumference of the Earth.