We usually use extensions when we feel that our data can be better displayed and with more features in a different manner besides the qlik vizualizations. You can try exploring different extensions present in the Qlik Branch.(qlik.branch.com)
Extensions are created by developers who feel they can represent their data in a better fashion.hence you can find it that way too.
It moves away from the usual qlik visualizations that everyone uses, hence it might look good.
Since it is not supported by Qlik, it might not work properly with your project.
It might not get updated or stay working as qlik releases the versions.
Since it is pre defined, you might not find it suitable for your project or will not be in a position to add features.