Twig tag include vs function include

{{ include() }} Was introduce in Symfony 2.2:

Using a function allows you to do whatever you want with the output (which is not possible with a tag), like a simple:

{{ set content = include('some_template') }}

But as Fabien Potentier (twig founder) said

the function and the tag does indeed the exact same thing

You can find the discution about it's introduction here: https://github.com/twigphp/Twig/pull/926