It's about the control you can have. You can gradually add in annotations without having to rewrite the whole program as well as being able to selectively check

It's about the control you can have. You can gradually add in annotations without having to rewrite the whole program as well as being able to selectively check buffer-overflows (i.e. checking everything vs checking code regions that you don't have security guarantee but would like to have). Additionally they also have features that allows you check overflow statically on limited cases.