Transitions are are good way of showing images of the same height and width one after the other within the same space. This could decrease the footprint of a web page considerably.
They can either be of the same subject matter, but in different states, or completely different images altogether.
They can be very affective and visually stimulating to the eye and can add added interest to a page and makes you want to read the text with the same page. If you are reading this then that is proof enough!