The semantics are different with regard to lazyness: a boxed vector can contain thunks (unevaluated values), so individual elements can be evaluated on demand,

The semantics are different with regard to lazyness: a boxed vector can contain thunks (unevaluated values), so individual elements can be evaluated on demand, while an unboxed vector must be evaluated all at the same time (making it difficult to run a transformation in parallel with multiple threads, for example).