symfony/symfony-docs — comment on commit 78fbe0505f42b091eca4dd42b780291e3eed950d
File: cookbook/form/create_custom_field_type.rst line 110
By stof:

You would need it here because you want to use the block with the same name in another theme.

When using the form rendering, you can however access all blocks defined in the templates by using ``block()`` due to the way they are handled.