Why std::vector iterator is invalidated after the erase() call?

The C++ reference clearly state that calling std::vector::erase(it) on iterator will invalidate all iterators pointing to and after the erased element. http://ift.tt/1tl2KTy

I do understand why such iterators became non-dereferenceable after the erase call, but i am curious why they need to became invalid, what implementation details require it?

For instance standard says that std::vector must be implemented with elements stored contiguously and that elements can be accessed not only through iterators, but also using offsets on regular pointers to elements so it seems logical that iterators for such container will probably be implemented as pointers - but then how pointers could became invalidated?