A somewhat contrived example which may require a quick look at the API docs but in the main you can focus on solving the problem, premature optimization and all that.
How many people need to implement a lang runtime?, less than 0.0001% of all devs I would imagine. They can use C or C++ which has done the job for decades.
When you've worked with languages with a garbage collector, having to worry about memory management and all that jazz seems a step backwards just to save a few cycles. You want to be focusing on the domain problem not the machine. (This is directed at people who use Rust as a general purpose lang, im sure it's good for low level systems programming).
This could be a prelude to Apple offering their own Cloud services like AWZ or Azure, they need a standard backend Language that runs on traditional server OS's.
Regardless being able to implement backend and apps in one language is huge.