My guess is that it's for efficiency. You have to dynamically allocate a string and then copy it to the destination if you separate "format" and "output". If yo

My guess is that it's for efficiency. You have to dynamically allocate a string and then copy it to the destination if you separate "format" and "output". If you combine them, then you can directly output during formatting to your destination. Eliminating this inefficiency while separating "format" and "output" requires some sort of lazy interface or language hack.