Modal windows are very useful when trying to add some effects to some elements of a website. They help to give a neat and nice structure and effects to things like contact forms, lightbox for videos and images or even text, signup and login forms, search boxes, alert or notification boxes, transitional elements and much more. Check out these great examples of the use of modal windows.
Here are a few use and applications of modal windows.
Alerts and Notifications
Help and Tips
Blur and Fade Out Background of Window