That's a very good question.

My understanding is that the main motivation behind the introduction of iostream was to make a type-safe variant of printf. So with iostreams the _type_ of the value is determined at compile-time, but the representation (e.g. should it be stringified as dec or hex? should it be left-align or right?) needs to be determined at run-time.

In other words, the design issue is only partially solved with iostreams in terms of performance.