In high school classes, development is often taught in terms of answering Wh- questions. This is a way of making sure you've explained yourself. College level assignments usually call for telling why and how. This is also often described as using support, providing examples, giving evidence, backing up your opinions, including specifics, etc. Having to develop your ideas (or, in the case of summary, someone else's) ensures that you think them through and shows the reader you've done so. This is also the basis for persuasion.