Writing styles are divided into several groups in the United States, depending on the final form. For example, the Associated Press (AP) style is straight-forward and tries to make everything as clear and concise as possible. This style is used by newspapers nationwide. APA, or American Psychological Association style, on the other hand, delineates how papers, such as those for college or professional usage, are to be created for consistency for those fields.
Modern Language Association (MLA) style is used largely in high schools for research papers. It sometimes is used in colleges, too. It defines how to reference research materials and how to write in the consistent style teachers are familiar with.
AP style is used for newspapers because they are designed to be read by everyone, not just scholars, scientists, engineers or lawyers. The style uses techniques that are transparent. For example, when the U.S. Postal Service shortened all the states' names to two letters, it led to confusion such as with Maine. The postal form is ME, but AP says to spell out any state or month of five letters or less. Unnecessary words also are cropped out. For example, "She said that she was leaving." AP dictates the removal of "that" because the sentence doesn't need it. Other, less obvious, variations include the removal of "doctor or Dr." from the beginning of anyone's name if the person is not a medical doctor. Most of the time, the honorary title is irrelevant.
APA style is used largely for college course work. The reason, according to the organization's website, is to have a consistent style for "punctuation and abbreviations, construction of tables, selection of headings, citation of references, presentation of statistics as well as many other elements that are a part of every manuscript." Professors who are grading college papers see hundreds of them each year. Without guidelines, the references might come in a different format for every paper. APA style also delineates formats for tables and charts for the same reason.
The Chicago Manual of Style has dictated manuscript style since it first was published by the University of Chicago Press in 1906. Used largely for book manuscripts, it gives specific rules for outlining, book structure, and footnotes and citations. It is a bit more flexible than APA because it allows the mixing of formats.
All these styles and others are used for consistency in writing. Like Daniel Webster, who wrote the first American English dictionary, the idea is to make writing clear, concise and consistent so that those reading a book or a newspaper has a basis from which to start. This is despite the fact that readers rarely consciously notice what style is being used. Writing is held to a standard, like many other professions such as medicine, and style guides gives writers guidance as to how to achieve those standards.