This does two things (and possibly more in the future):
constdeclarations to primitive types (excluding Object/Array) at the top of a scope before anyletorvardeclarations will be inlined into their usages.letandconstdeclarations only used once are inlined into their usages.
JavaScript engines typically do these optimizations internally, however it might only happen much later in the compilation pipeline, after code has been executed many many times.
This will typically shrink the output size of code, but it might increase it in some cases. Do your own benchmarks!