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There is a need to interrupt current process and catch focus the user’s full attention to something really important. (For example to notify the user about some major changes to your service since the last login, tell the user that his login has expired and provide a login form to let him login again, warn the user about doing something that is not reversible etc.)
You need to display user actions or other options that are “independent” from other pages.
Placing them in an overlay saves the user a return trip to another page and makes it easy to get back to the original page where he came from.
It is important to show additional or related content/options in context
Solution
When a dialog box or modal window appears on the screen, an action must be taken relating to it before you can access other applications on the computer. This focuses user attention on important information or performing an action