Corrected:
There are a lot of potential reasons to use something like this, but having a main selling point of it being "lightweight" is just flat out lying or willfully misunderstanding what lightweight means.
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Javascript, and all scripting languages in general (maybe excepting lua and similar) are fundamentally not lightweight. They require enormously complex runtimes.
I'd say a reasonable definition of "lightweight" for an application runtime is if a complete, distributable app including all the required components can fit in 1 MB (This functionally excludes things like .NET too).
There are, or course, odd corner cases (using the OS UI components, etc...), but it's broadly applicable.
Other relevant bits to being lightweight - how fast is the application to start/stop? What's the minimum runtime overhead? etc...