I think a vec is important for the same reason a string is… because being able to properly get the length, and standardized ways to push/pop from them that don’t require manual bounds checking and calls to realloc.

Hash maps are mostly only important because everyone ought to standardize on a way of hashing keys.

But I suppose they can both be “bring your own”… to me it’s more that these types are so fundamental and so “table stakes” that having one base implementation of them guaranteed by the language’s standard lib is important.