I think by far, the most common use-case is going ...

# stdlib

[kevinmost]

I think by far, the most common use-case is going to be combining collections (either a union of two sets, or concatenating two lists)

[karelpeeters]

I don't really know how immutable collections are implemented/used, but I image they would be useful in something like flatMap, which always concatinates lots of lists together.