Clarification of Immutable and Mutable Objects in Scala
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The way I see it: there are two kinds of entities here: variables, and objects these variables reference/point to. If a variable is mutable, it can be made to reference a different object. If it is immutable, it will always reference the same object, the one it initially references. If an object is mutable, its properties/contents can be changed (e.g. adding to an array). If it is immutable, it will always remain the same as it was when it was initialized.
If the data you have is not going to be written to often (not never, mind) but will be shared widely, then it makes sense to use an immutable object (or structure) which will allow the shared aspects to vary locally, but minimize copying and redundant memory usage globally.
If you're going the other way, frequently written to and not shared widely, then mutable structures have advantages since mutation can be faster than partial reconstruction.
Not to say that this always holds true, but it's a decent rule of thumb and can help you make sane choices when initially selecting collections.