I couldn't find any clear online material that could explain what it is useful for.
It is reserved, but not necessarily used (well, not at least of time of writing).
The following are reserved as future keywords by the ECMAScript specification. They have no special functionality at present, but they might at some future time, so they cannot be used as identifiers. (Note that for the moment Mozilla reserves these keywords only in strict mode code. Most other browsers reserve these keywords in all code, whether strict or not, so their use is very much non-portable. Mozilla will likely reserve these keywords in normal code in a future release, to conform to the specification and to be consistent with other browsers).
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