The documentation on iterators describes the advantages of using iteration rather than direct indexing in a loop: http://doc.rust-lang.org/nightly/book/iterators.html

The gist is that an iterator has enough information about the thing that it's iterating over that we, as library authors, can avoid unnecessary bounds checking and just perform unchecked indexing while retaining all the safety. LLVM is also surprisingly good at automatically vectorizing our iterators.