Your question is a bit like someone asking "so what does the garbage collector actually do? Does it X or Y? What impact does it have?"

And the answer is: no need to care about it. Unless you need to really optimize for high performance (not necessary in 99% of the cases, otherwise you'd use a different language from the beginning anyways).

> Also, the original strong need for immutable data in the first place is safety under concurrency and parallelism?

One of the reasons is that you really can just completely stop thinking about it. Just like you can stop thinking of (de)allocations. Except for some edge-cases when performance matters a lot.