Standard Java Jakarta Enterprise Edition (Jakarta EE) applications use JavaServer Pages (JSPs) to generate presentation views for the end user. JSP is a mature technology that enables the use of embedded Java code as well as JavaServer Pages Standard Tag Library (JSTL) elements, which will, in turn, execute Java code, which can generate presentation views. All JSPs are eventually compiled as a Servlet.

But mixing these Java codes and presentation-specific codes (HTML, CSS, and many more) is cumbersome and makes the separation of concerns difficult. Furthermore, presentation views mad using plain JSPs are difficult to modify and be maintained by UI engineers. That is where UI template engines are useful. 

Template engines provide an easy way of decoupling presentation view code from business logic so that each is layered and can be developed and maintained independently of the other. This helps vastly reduce code duplication and the number of bugs introduced because template engines allow the reuse of previously tested, production-ready code.