If you mean over C-style arrays, I'd say it's ease of management. I remember when I wrote my first solution for TC problem I tried to use C-style arrays - and then a friend showed me vectors, and I was immediately impressed by how much shorter and cleaner my solution became. C-style arrays are fixed-length, and you have to keep track of their actual length (if you're generating elements one by one up to certain quantity), to worry about how much you should allocate beforehand, to handle pointers when passing them as params and when having them multi-dimensional. vectors can be resized at will, have lots of useful algorithms in STL (sorting, search, removing duplicates etc.) and are generally easier to handle.
