Writers might do this to develop their plots, themes, or perspectives. Perhaps one of the masters of this is William Faulkner, whose novels moved across decades and characters. Although this certainly makes Faulkner a challenging read, the result is that Faulkner's books have created not just a single story, but a whole tapestry of characters, communities, and times. As readers, we see how these characters and lives intersect.
Other modernist writers also employed this technique to demonstrate the tenuousness of what we might call "truth." By manipulating perspectives and settings, modernist writers called into question a linear understanding of the world. In other words, this technique works to support the larger theme of an unknowable universe.
Of course, this technique is very common, and is used for many reasons. Another way this is useful is to build suspense. In thriller, noir, and mystery novels, writers can show the readers what various characters are thinking and planning. Thus, we as readers are privy to information the heroes are not. This can add to the suspense as we read. We think to ourselves, "No, no, no! Don't open that door! It's a trap!"
It's possible that the author feels that in order for the book to have more debt and feeling, he or she creates a character shift. Having a setting shift also helps the reader to feel he or she is getting more information than from just one point of view. You see this also many times in movies. We can see from one characters point of view, however until we see from the other character, we don't truely understand.
The effect this has on the book, is that it creates an opportunity for the reader to gain a better understanding of what the plot is.
You probably need to be a little more specific in you question to get a useful answer.
As a writer, I do this for a variety of reasons. Other authors probably do this for a variety of OTHER reasons--or some that are the same.
If you mean: why does a specific author change setting, character, POV, etc., etc. in a specific work, that's a much easier question to answer (sometimes).