iPhones have never given users the option of expanding storage with a microSD card. As the generations have advanced over the past few years, native storage options have increased, but for many, the 16GB iPhone was the affordable option as well as the practical one.
16GB may suit a lot of people’s needs. It is enough space for a movie or two, several hundred songs (or a few thousand if you forego the movies), and a lot of pictures. However, people who “think” 16GB will be enough space often underestimate just how much media, usually pictures, they will accumulate.