Liferay supports the use of Freemarker in fragments in order to provide fragment developers with a strong templating system to create the HTML code of the fragment. Using freemarker is not mandatory and not necessary for simple fragments, but it's an option always available for developers.
Similarly, Freemarker is also supported to create Web Content Templates and Widget Templates (aka ADTs).
In some scenarios, however, it's not desired to give this possibility to fragment developers since freemarker code can potentially have a negative impact the system since it's so powerful.
The goal of this feature is to give System Administrators the option to disable the usage of freemarker across all virtual instances of the installation, while limiting the provided functionality as little as possible.
For example, an option could be provided to disable the use of freemarker in any of the fragments created via the fragment editor UI or imported. Freemarker should still be valid for fragments provided out of the box or using the fragment contribution mechanism, since those go through the same deployment and approval process as any other backend code.