Even if it didn't dedupe strings, mutable string literals means that it has to create a new string every time it encounters a literal in run time. If you have a literal string in a method, every time you call the method a new string is created. If you have one inside a loop, every iteration a new string is created. You get the idea.
With immutable strings literals, string literals can be reused.
You make an arrow function that takes an object as input, and calls another with a string and a field from the object, for instance to populate a lookup table. You probably don’t want someone changing map keys out from under you, because you’ll break resize. So copies are being made to ensure this?
With immutable strings literals, string literals can be reused.