Why do mobile apps use animated transitions?

So many mobile apps are using animated transitions (much like the web was using flash animations in the early 2000s). In a desktop internet browser, clicking a link on a web page takes you to the the next page while a lot of mobile apps will fly screens from different directions. When is it okay to use animated transitions and do mobile apps need these transitions?

(For context, Snapchat uses almost no animation transitions successfully and creates a quick and responsive experience. I wonder if this will become the norm.)