Julia and Swift are different enough that there is benefit in having both with that functionality. Julia is a dynamic and heavily interactive language (especially when it improves more JIT latency and "time to first plot") for exploratory analysis (like what Python provides) and development without compromising performance in any step, with strong focus on HPC/Distributed computing.

Swift has some interactivity, but it's core execution strategy is to create static binaries to be deployed in mobile and desktop, with much better support for high quality end user interfaces (especially in apple devices).

While as a researcher and mostly linux developer I will probably not use Swift, I can easily see situations in which Swift would be the clear choice given both Zygote and Swift for Tensorflow equally mature, even when Julia's scientific computing support is much broader.