Our own world looks like it is a sphere at first glance, but when you actually measure its dimensions it turns out our planet is actually an oblate spheroid. Why is this important? The shape of the Earth along with other aspects of the Earth’s rotation causes differences and changes in climate, the night day cycle, and meteorological phenomenon. Also this is a random fact that you may or may not know.
In the same way the shape of your world will affect many things that happen within it. Your world does not need to be round or even a planet. Your world can be anything such as a space station, two giant creatures in the middle of a flat ocean, or even a turtle with 4 turtles and a disk on its back flying through space.
But the actual shape of your world and what the people who live within your world think its shape is may differ. For centuries people believed that the Earth was flat. Something similar may be going on in your world. Usually this happens because technology has not advanced enough to detect the shape of the world. However if your world is strangely shaped like the Bionis and Mechonis from Xenoblade, or like a space station it would be obvious to the people living in them to know their shape.
This is just something simple to consider that can mean a lot when you are building a world.