A sphere is a geometrical shape that contains a maximum volume, in this case mass, which has gravity, with the the least amount of surface area. A sphere is the most economical and efficient packaging of mass.
Yes, if planets or stars revolve fast enough, they are "flattened" proportional to their rate of spin. Because the Earth currently has more water in the southern hemisphere, the Earth's rotation turns it into an oblate spheroid, not a perfect sphere, especially when you take into account differences in elevation and plate tectonics.
