When Symfony1 was created, the framework did not use any of existing template systems but relied on just the PHP as a template language. It had some advantages (that is, templates did not require any processing and were very fast), but wasn't too friendly for frontend developers. Also, PHP is a very verbose language and you need to write a lot of code to do some basic stuff such as checking a variable and determining whether it exists or is empty. Also, escaping variables requires some coding.
In Symfony2, a new templating engine, called Twig, was introduced. Twig was written with a few concepts in mind, such as:
Templating system is extensible: Developer may write their own extensions to it very easily.
Fast: The written template is compiled and processed, generating an optimized PHP code on the output.
More secure: It is much easier to make code secure using Twig with features such as auto-escape or sandboxing.