I believe that they've framed it in this article as mostly for compatibility with C/C++. A simple example is JS objects are often represented as a single f64 wi

I believe that they've framed it in this article as mostly for compatibility with C/C++. A simple example is JS objects are often represented as a single f64 with the NaN space punned for 32-bit pointers and 53-bit integers. If you want to put some code written in Rust in that environment, if you could define a union that could expose a safer API by using bit twiddling you could. Presumably if you have the memory to spare or are unconstrained in design you won't use this feature which is part of why it took so long to get into the language.