It could make refactoring and debugging easier. For instance, let's say you're part way through some operation and it fails, and the error recovery logic is buggy and leaves the data structure in an inconsistent state. You could debug the problem, but if you're in a hurry and you just need something that works right now, you can store a reference to the data structure at the beginning of the operation and revert to that if anything goes wrong. You'd pay a performance cost since all the direct in-place edits would turn into copies and allocations, but it would at least work.

Similarly, you could develop a program initially in an all-immutable multi-owner style, and then selectively convert codepaths to single-owner style to get better performance in the parts where performance matters.

Also, shared data are much easier to reason about if it's immutable (not necessarily even in a concurrent execution context).

For instance, if you write an interpreter, representing variable bindings with a functional map allows to reason about scope super easily; when you change scope, you just create a new map with the new variable bindings, and when the scope ends you just get back the map at the scope beginning which is still valid.

With a stateful hashtable, you need to undo the changes you did in you scope; the value of the table when opening the scope that you want to get back to is gone forever. Now it ties you program to a certain execution order. One way to see that is that if you want to collect environment to each scope in some data structure for some reason, you need to make deep copies everywhere.

It impacts the performance profile of the collection, and thus isn't merely an implementation detail.