The user is changing or narrowing their focus of operation, completing a command, opening a new area in the interface or they need to know that application state is changing. The designer wants to keep the user aware of a change in relationship between elements, a change in application state, a way to slow down an interaction, or make time seem to pass faster.
Designer needs to communicate that an object is changing its spatial relationship within the page.
Designer wants to make a previously dimmed object a point of focus by making the object bright again.
The designer needs to communicate that an object is no longer of primary importance, yet wants to keep it available in a smaller form.
Designer wants to communicate that a new view of an object is replacing the previous view of the object.
Designer needs to communicate that an object is of secondary importance.
Designer needs to show the detail of an object in its context or reveal a previously collapsed object.
Designer needs to communicate that an object is being added to the page or application.
Designer needs to communicate that an object is going away.
Designer wants to show that an object has been removed from a list of objects.
The designer wants to bring new content into the page and would like to communicate the additional content's relationship with other items on the page.
Designer needs to call attention to where a data value or content has changed within the interface.
Transitions are a powerful way to communicate change in a user interface. Transitions can communicate a change in relationships between elements on a page or a change in an element's state. Additionally, when interactions are complex or involve important changes transitions can be used to slow down an interaction allowing the user to "see what just happened." During lengthy processes, transitions can be used to make time seem to pass faster by distracting the user during the waiting time.
In the real world most things don't just appear or disappear immediately. They might fade, wither, grow or fly away. Yet in most user interfaces little consideration has been given to making objects operate more like the real world.
Transitions are a communication device. Having said that, it is important that the goal of communication be kept in mind regarding any transition effects.
In more engaging or entertainment based sites a transitional effect can become more important and thus may require longer transition times or more effects. In communication-oriented web sites, the transition is secondary and is only communicating changes in the interface.
From the discussion above we can extract some general rules.
Sudden movement or or change in lighting is processed by a second region in the brain that is dedicated to visual processing. Besides the visual cortex which processes what we normally think as vision, the superior colliculus gets a copy of the same visual information. This region of the brain is dedicated to attention capture. Its main purpose is to process sudden changes in the visual space that might indicate danger or importance. It guides reflex movements telling the body to orient toward the change .
Since transitions are about visual communication they by default are not accessible by users with visual impairments. If what the transition communicates is important for the user not to miss then you can consider these techniques.