Why do we use template engine in web application?

I know this is very common question and before asking it i have searched on google but no one could convince me or I am not using rights keywords for my search.

I read somewhere that template allows you to refresh portion of the page where you want to make changes just like ajax but as I experienced it does not instead it loads whole page. So i have few questions

1) Does it really refresh portion of the page? If yes then how it works?

2) Why should I use template engine instead ajax for dynamic content loading?

3) What are the actual benefits of using template engine?

4) Which template engine I should use for better performance with struts application.