The promise of Julia is that you can have the high-level interface and the low-level code in the same language. The alternative would be coding the low level code in Rust or C and then creating bindings for Python or R.
For a while Julia made the most sense for long-running code that is that is executed almost as often as it is modified (e.g. scientific computing). In this situation Rust or C static compilation times become a hinderance. As ahead-of-time and static compilation features get added to Juliaz this scope will expand.