One of the most important features of jQuery is the ability to add animations to changes you are making to elements. This gives the user the feel of a slick, well-developed application rather than a clunky, traditional web page.
This is especially true if you are moving, resizing, or dynamically adding elements to the web page. It is frustrating as a user to all of the sudden have a bunch of new things appear or disappear. Using transitions gives the user a chance to see where elements are expanding from, shrinking to as well as the direction of movement so he can adjust his mind-set to accept the changes.