We always look up at the ceilings of places that we visit, especially if the building has a dome. Many of them are in cathedral, but not all of them. Throughout our history, people have been fascinated with the sky and the stars and often imagined what might exist up there. In many cases they use the ceilings to paint what they imagine might be beyond the clouds that hovered over their heads.
In other cases, artists just used the dome ceilings to create works of art that tell a story that changes with the direction you face as you look up at it. Sometimes the artist will just use the geometrical shape to create a unique look inside of the dome. Regardless of the style of artwork, looking up at interior of domes is something that is worth taking the time to pause and enjoy.