The duty of a template engine is to separate PHP code from HTML content (design). The PHP code will be in different files from those files that contain HTML content. A first advantage is that a site design (layout) can be done independently of the programming (business code).
Let's see some of the advantages of using Smarty:

1) The loading of the website is improved
Smarty can cache every template and design and and because of that will load much faster.

2) Keep the design clean
Because Smarty it is based on templates, this can be a great advantage to keep the design clean for the entire project.

3) Separation of code design
When using Smarty you can separate the frontend from the backend so developers will not disturb the programmers and vice versa.The PHP code will be separated from everything that is XHTML and CSS.

4) Front-end speed is improved

Since many times the templates are improved and added new options for users, Smarty can be the perfect solution for this. The more you add new features (using jQuery, Ajax or other) the more you will notice that when using Smarty everything is simple and does not damage the existing application.

5) Very high maintainability
With Smarty is possible to have a set of templates that you can reuse them whenever needed. Every design change to your website can be done in the shortest time.

 6) High Security
Designers and programmers will work separately and this is an advantage because is a very low probabilty to happen some accidents that would affect the design or the application code.