As you can read from the tutorial,
The term Facelets is used to refer to
the JavaServerTM Faces View Definition
Framework, which is a page declaration
language that was developed for use
with JavaServer Faces technology. As
of JavaServer Faces 2.0, Facelets is a
part of JavaServer Faces specification
and also the preferred presentation
technology for building JavaServer
Faces based applications.
Now, JSP is the standard view technology for the web. But when it comes to adopt JSF as a web framework, JSP doesn't support all the features provided in JSF 2.0. Therefore, there was a need of some standard view technology to work with JSF 2.0. So, Facelets is just that.
If you are not working with JSF at all, you don't need to employ Facelets. You can go with JSP in that case. Or there are few other template frameworks, like FreeMarker, and Velocity. Moreover, some web frameworks, defines there own mark-up, like Struts2. So if you decided to go with Struts, using Struts mark-up is the way to go.
