pre-defined libraries for writing html code makes it easy to implement it
Struts uses JSP for the templating mechanism, so one would rarely write HTML, but JSP. Struts has JSP tags that are used to implement forms (and a few other things), but that's true of most Java web frameworks, so it's not a distinguishing characteristic of Struts.
maintainability would automatically come if i am using MVC
Maintainable code is about much more than using MVC, and no framework can help you if the code you write doesn't follow good practices.
i went through many sites searching for the answer but i think its only because of the MVC implementation
Struts does much more than MVC, as do almost all other Java web frameworks, and its user guide will tell you what all those things are. Be aware that Struts 1 is dead for all practical purposes; new projects should not use it. And Struts 2 is different in an incompatible way, and thus very unlikely to be used as widely as Struts 1, since today there are many more frameworks available than there were when Struts 1 became popular.