Perhaps if you looked at the output they generate and the attributes they used you'd have more of an idea? I'd say the form binding and error messages are the big ones.
The values automatically get filled for you if there is a FormBean property matching the name of the form element (like if I have an FormBean property age and a html:text on the JSP page with the name age, then the value of age in FormBean will automatically be filled into the text field) and you automatically get error messages if validation fails...