Because it's not merely mutable, it's exclusive. You get a static guarantee that, for as long as you can use this reference, this is the only reference in the entire program that can mutate this memory.
This is automatically thread-safe, without any locks. It's guaranteed that there can't be any side effects that could affect this memory, no matter what code you call. You don't need any defensive coding copying the memory just in case. It optimizes well, because it's certain that it won't overlap with any other region.
C++ doesn't have that kind of strong no-alias guarantee. Even memory behind const pointers can be mutated by something else at distance. The closest equivalent is C's restrict pointers, but they're more coarse-grained, and aren't checked by the compiler.
This is automatically thread-safe, without any locks. It's guaranteed that there can't be any side effects that could affect this memory, no matter what code you call. You don't need any defensive coding copying the memory just in case. It optimizes well, because it's certain that it won't overlap with any other region.
C++ doesn't have that kind of strong no-alias guarantee. Even memory behind const pointers can be mutated by something else at distance. The closest equivalent is C's restrict pointers, but they're more coarse-grained, and aren't checked by the compiler.