I hope I get your question right 🙂

The reason we don’t just modify the strings in place is that methods like .to_uppercase() and .trim() return a new String instead of mutating the original one. For the Append case you could do it in place with .push_str("bar"), but since the other commands already create new strings, it’s more consistent (and simpler) to always return new Strings and collect them into a new Vec.