Why is Earth a sphere?
Our planet, Earth, has a spherical shape. However, it is not a perfect sphere and the presence of valleys and mountains make it somewhat rough. In addition, the Earth has slight bulge near the equatorial region and has more mass there than polar regions.
The shape of Earth is governed by its mass and gravity. When the Earth formed billions of years ago, it was a rotating mass of gases and dust. Over time, as more and more mass was added to this spinning disc of gas and dust, gravity started pulling everything towards it. The most efficient surface according to gravitational force is a sphere, since all the points on a sphere are equidistant from the center. As the mass of Earth continued to grow, gravity also increased and attracted everything in its reach. And thus Earth came to be spherical. However, the slight distortion in its shape is due to its rotation on its own axis. This rotation creates centrifugal forces, which cause Earth to be slightly fatter at the equator as compared to the poles.
Hope this helps.