This type of writing enables students to demonstrate their knowledge of a subject. When teachers assign research papers or essays, they are employing WID. For example, a history teacher who assigns an essay on the causes of the American Revolution is encouraging students to summarize what they have learned and to communicate it in a coherent manner. A science teacher who assigns a research paper that requires students to research a disease and its treatment can see whether students are able to synthesize material from various sources and present it in the correct form.
Supporters of WID often point out that various academic subjects require different types of writing conventions and forms, and that WID helps students identify and use them correctly. For example, the passive voice may be eschewed in an English paper, as would APA style. In a Science or Psychology paper, however, both would be acceptable. Therefore, writing across the disciplines enables students to learn more about the field of study, including what is considered acceptable for writing in that field.
Some teachers use only one of these two approaches, and some use both. Whether you're using WTL or WID, writing across the curriculum can be a helpful way to incorporate writing into every subject you teach.