Accepted answer

facets automate some parts of project configuration and deployment.
for example inform you when servlet is not added to web.xml in dynamic web project. stuff like that.
when you add jpa facet, eclipse will create persistence.xml and will keep notifying when you create entity class but don't configure it in persistence file.
there's lots more, adding facets to projects also reconfigures how your project structure looks in eclipse.

basically they do exactly as you quoted: "

adds support for writing applications using java programming language.

every facet add something new so if you want more specific answer you have to answer question about specific facet. if you don't know what they can do for you - turn them off. you can always add them when you learn more.

real life example:
adding jpa facet messed up my project using objectdb by creating persistence file, which, turned out, i didn't even need.

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